Do not run any X server on sys-gui-gpu

On sys-gui-gpu we use lightdm for running X server, but we still need
guivm-gui-agent service

appvm-scripts/usrbin/qubes-run-xorg

@@ -105,7 +105,11 @@ if [ -x /bin/loginctl ]; then
     loginctl attach "$XDG_SEAT" /sys/devices/platform/pcspkr/input/*
 fi
 
-if qsvc guivm-gui-agent; then
+if qsvc lightdm; then
+    # case: sys-gui-gpu
+    exit
+elif qsvc guivm-gui-agent; then
+    # case: sys-gui
     DISPLAY_XORG=:1
     DISPLAY_XEPHYR=:0
 
@@ -119,6 +123,7 @@ if qsvc guivm-gui-agent; then
     # Run xsession into Xephyr
     /usr/bin/qubes-gui-runuser "$DEFAULT_USER" /bin/sh -l -c "DISPLAY=$DISPLAY_XORG /usr/bin/xinit $XSESSION -- /usr/bin/qubes-run-xephyr $DISPLAY_XEPHYR > ~/.xsession-errors 2>&1"
 else
+    # case: non-gui domain
     # Use sh -l here to load all session startup scripts (/etc/profile, ~/.profile
     # etc) to populate environment. This is the environment that will be used for
     # all user applications and qrexec calls.