Fix VM autostart - set $DISPLAY env variable

Without this, started qrexec-daemon would not have access to GUI,
especially can't display Qubes RPC confirmation dialogs.

linux/systemd/qubes-vm@.service

@@ -4,6 +4,7 @@ After=qubes-netvm.service
 
 [Service]
 Type=oneshot
+Environment=DISPLAY=:0
 ExecStart=/usr/bin/qvm-start --no-guid %i
 Group=qubes
 RemainAfterExit=yes