qubes-dom0-update: Pass --check-only option to the updatevm too

The updatevm script does need this option too, for two reasons:
 - to skip downloading updates if only check was requested
 - to report updates availability via exit code

The second point is critical for setting 'updates-available' flag.

Fixs QubesOS/qubes-issues#7417

dom0-updates/qubes-dom0-update

@@ -82,6 +82,7 @@ while [ $# -gt 0 ]; do
             ;;
         --check-only)
             CHECK_ONLY=1
+            UPDATEVM_OPTS+=( "$1" )
             ;;
         --preserve-terminal)
             exit_cmd=