vm/dom0-updates: add --gui switch as oposite to --nogui (#354)

Dom0 can now control whether to use GUI or no - not relying on script default.
Actually it is used to cancel --nogui passed earlier.

common/qubes_download_dom0_updates.sh

@@ -16,6 +16,9 @@ while [ -n "$1" ]; do
         --nogui)
             GUI=0
             ;;
+        --gui)
+            GUI=1
+            ;;
         --clean)
             CLEAN=1
             ;;