system: vm-image.tmpl: Add a crutch to allow refreshing the resolution.

Inspired by
1270b2e209.
The price to pay is about 5% of a CPU core usage in the guest in the worst
scenario.

* gnu/system/examples/vm-image.tmpl
(auto-update-resolution-crutch): New mcron job.
[services]: Extend the mcron-service with it.

gnu/system/examples/vm-image.tmpl

@@ -5,7 +5,7 @@
 ;;
 
 (use-modules (gnu) (guix) (srfi srfi-1))
-(use-service-modules desktop networking spice ssh xorg)
+(use-service-modules desktop mcron networking spice ssh xorg)
 (use-package-modules bootloaders certs fonts nvi
                      package-management wget xorg)
 
@@ -24,6 +24,18 @@ Run '\x1b[1;37minfo guix\x1b[0m' to browse documentation.
 accounts.\x1b[0m
 "))
 
+;;; XXX: Xfce does not implement what is needed for the SPICE dynamic
+;;; resolution to work (see:
+;;; https://gitlab.xfce.org/xfce/xfce4-settings/-/issues/142).  Workaround it
+;;; by manually invoking xrandr every second.
+(define auto-update-resolution-crutch
+  #~(job '(next-second)
+         (lambda ()
+           (setenv "DISPLAY" ":0.0")
+           (setenv "XAUTHORITY" "/home/guest/.Xauthority")
+           (execl (string-append #$xrandr "/bin/xrandr") "xrandr" "-s" "0"))
+         #:user "guest"))
+
 (operating-system
   (host-name "gnu")
   (timezone "Etc/UTC")
@@ -89,6 +101,9 @@ root ALL=(ALL) ALL
                  ;; integration with the host, etc.
                  (service spice-vdagent-service-type)
 
+                 (simple-service 'cron-jobs mcron-service-type
+                                 (list auto-update-resolution-crutch))
+
                  ;; Use the DHCP client service rather than NetworkManager.
                  (service dhcp-client-service-type))