scripts: system: Rationalize persistency.

Make sure that the images are created with a non volatile root by default and
the vm are created with a volatile root by default. Break the --volatile
option into --volatile-image and --persistent-vm options.

* guix/scripts/system.scm (perform-action): Turn volatile? argument into
volatile-vm-root?.
(show-help): Introduce --volatile-image and --persistent-vm options instead of
--volatile.
(%default-options): Adapt it.
(%options): Handle those options.
(process-action): Honor them.
* doc/guix.texi (Invoking guix system): Adapt it accordingly.

doc/guix.texi

@@ -35162,6 +35162,11 @@ $ $(guix system vm my-config.scm) -m 1024 -smp 2 -nic user,model=virtio-net-pci
 
 The VM shares its store with the host system.
 
+By default, the root file system of the VM is mounted volatile; the
+@option{--persistent} option can be provided to make it persistent
+instead.  In that case, the VM disk-image file will be copied from the
+store to the @env{TMPDIR} directory to make it writable.
+
 Additional file systems can be shared between the host and the VM using
 the @option{--share} and @option{--expose} command-line options: the former
 specifies a directory to be shared with write access, while the latter
@@ -35199,8 +35204,8 @@ QEMU monitor and the VM.
 @cindex Creating system images in various formats
 @item image
 @cindex image, creating disk images
-The @code{image} command can produce various image types.  The
-image type can be selected using the @option{--image-type} option.  It
+The @code{image} command can produce various image types.  The image
+type can be selected using the @option{--image-type} option.  It
 defaults to @code{efi-raw}.  When its value is @code{iso9660}, the
 @option{--label} option can be used to specify a volume ID with
 @code{image}.  By default, the root file system of a disk image is

guix/scripts/system.scm

@@ -772,7 +772,7 @@ and TARGET arguments."
                          dry-run? derivations-only?
                          use-substitutes? target
                          full-boot?
-                         volatile?
+                         volatile-vm-root?
                          (graphic? #t)
                          container-shared-network?
                          (mappings '())
@@ -827,7 +827,8 @@ static checks."
   (mlet* %store-monad
       ((sys       (system-derivation-for-action image action
                                                 #:full-boot? full-boot?
-                                                #:volatile? volatile?
+                                                #:volatile?
+                                                volatile-vm-root?
                                                 #:graphic? graphic?
                                                 #:container-shared-network? container-shared-network?
                                                 #:mappings mappings))
@@ -998,6 +999,8 @@ Some ACTIONS support additional ARGS.\n"))
       --no-bootloader    for 'init', do not install a bootloader"))
   (display (G_ "
       --volatile         for 'image', make the root file system volatile"))
+  (display (G_ "
+      --persistent       for 'vm', make the root file system persistent"))
   (display (G_ "
       --label=LABEL      for 'image', label disk image with LABEL"))
   (display (G_ "
@@ -1080,7 +1083,10 @@ Some ACTIONS support additional ARGS.\n"))
                    (alist-cons 'install-bootloader? #f result)))
          (option '("volatile") #f #f
                  (lambda (opt name arg result)
-                   (alist-cons 'volatile-root? #t result)))
+                   (alist-cons 'volatile-image-root? #t result)))
+         (option '("persistent") #f #f
+                 (lambda (opt name arg result)
+                   (alist-cons 'volatile-vm-root? #f result)))
          (option '("label") #t #f
                  (lambda (opt name arg result)
                    (alist-cons 'label arg result)))
@@ -1149,7 +1155,8 @@ Some ACTIONS support additional ARGS.\n"))
     (image-size . guess)
     (install-bootloader? . #t)
     (label . #f)
-    (volatile-root? . #f)
+    (volatile-image-root? . #f)
+    (volatile-vm-root? . #t)
     (graph-backend . "graphviz")))
 
 (define (verbosity-level opts)
@@ -1219,7 +1226,8 @@ resulting from command-line parsing."
                                            ((docker-image) docker-image-type)
                                            (else image-type)))
                             (image-size (assoc-ref opts 'image-size))
-                            (volatile?  (assoc-ref opts 'volatile-root?))
+                            (volatile?
+                             (assoc-ref opts 'volatile-image-root?))
                             (shared-network?
                                (assoc-ref opts 'container-shared-network?))
                             (base-image (if (operating-system? obj)
@@ -1279,7 +1287,8 @@ resulting from command-line parsing."
                                #:validate-reconfigure
                                (assoc-ref opts 'validate-reconfigure)
                                #:full-boot? (assoc-ref opts 'full-boot?)
-                               #:volatile? (assoc-ref opts 'volatile-root?)
+                               #:volatile-vm-root?
+                               (assoc-ref opts 'volatile-vm-root?)
                                #:graphic? (not (assoc-ref opts 'no-graphic?))
                                #:container-shared-network?
                                (assoc-ref opts 'container-shared-network?)