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

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