Allow preloading without graphics

The service currently times out and later degrades the status of
is-system-running, but the GUI would never connect when it is disabled.

For: QubesOS/qubes-issues#1512

Author: ben-grande

appvm-scripts/qubes-gui-agent.service

@@ -9,6 +9,7 @@ TTYPath=/dev/tty7
 # custom PATH for X session can be set with ENV_PATH; otherwise service's PATH
 # is inherited
 #Environment=ENV_PATH=/usr/local/bin:/usr/bin:/bin
+ExecCondtion=test "$(qubesdb-read --default=True /qubes-gui-enabled)" == "True"
 ExecStartPre=/bin/sh -c /usr/lib/qubes/qubes-gui-agent-pre.sh
 ExecStart=/usr/bin/qubes-gui $GUI_OPTS
 # clean env