services: hurd-vm: Support persistent images again.

Fixes a regression introduced in bab6434f58
whereby following the example from the manual regarding non-volatile images
would no longer work.

* gnu/services/virtualization.scm (hurd-vm-shepherd-service): Pass ‘image’ to
‘system-image’ only if it’s an image.
* doc/guix.texi (Virtualization Services): Explicitly say that the ‘image’
field can contain a file name.  Remove use of ‘const’ for the ‘image’ field in
persistent image example and adjust text.

Fixes: guix/guix#4130
Reported-by: Maxim Cournoyer <maxim@guixotic.coop>
Change-Id: I1a81ce27fb45978a681310b8a8788ab671b3edf8

doc/guix.texi

@@ -40911,6 +40911,10 @@ The image type name.  Use @code{'hurd-qcow2} for a 32-bit image or
 The image object representing the disk image of this virtual machine
 (@pxref{System Images}).
 
+Alternatively, this can be the file name (a string) of an image, which
+is useful if you want the image to persist across service restarts---see
+example below.
+
 @item @code{disk-size} (default: @code{'guess})
 The size of the disk image.
 
@@ -41009,15 +41013,16 @@ with the following non-volatile secrets, unless they already exist:
 @end table
 @end deftp
 
-Note that by default the VM image is volatile, i.e., once stopped the
-contents are lost.  If you want a stateful image instead, override the
-configuration's @code{image} and @code{options} without
-the @code{--snapshot} flag using something along these lines:
+Note that by default the VM image is volatile---i.e., once the service
+is stopped, the contents are lost.  If you want a persistent image
+instead, override the configuration's @code{image} and @code{options}
+without the @option{--snapshot} flag using something along these lines:
 
 @lisp
 (service hurd-vm-service-type
          (hurd-vm-configuration
-          (image   (const "/out/of/store/writable/hurd.img"))
+          ;; Specify an out-of-store, persistent image.
+          (image "/out/of/store/writable/hurd.img")
           (options '())))
 @end lisp
 

gnu/services/virtualization.scm

@@ -1904,7 +1904,9 @@ is added to the OS specified in CONFIG."
                              '())
                       #$@net-options
                       #$@options
-                      "--hda" #+(system-image image)
+                      "--hda" #+(if (image? image)
+                                    (system-image image)
+                                    image)
 
                       ;; Cause the service to be respawned if the guest
                       ;; reboots (it can reboot for instance if it did not