dispvm: do not require shmoverride loaded to start gui daemon

This isn't needed anymore because we'll show no window in invisible
mode. This allows to prepare DispVM from firstboot.

core-modules/000QubesVm.py

@@ -1715,15 +1715,20 @@ class QubesVm(object):
             # Run GUI daemon in "invisible" mode, so applications started by
             # prerun script will not disturb the user
             extra_guid_args = ['-I']
+        elif not os.path.exists('/var/run/shm.id'):
+            # Start GUI daemon only when shmoverride is loaded; unless
+            # preparing DispVM, where it isn't needed because of "invisible"
+            # mode
+            start_guid = False
 
-        if self._start_guid_first and start_guid and os.path.exists('/var/run/shm.id'):
+        if self._start_guid_first and start_guid:
             self.start_guid(verbose=verbose, notify_function=notify_function,
                             before_qrexec=True, extra_guid_args=extra_guid_args)
 
         if not preparing_dvm:
             self.start_qrexec_daemon(verbose=verbose,notify_function=notify_function)
 
-        if start_guid and os.path.exists('/var/run/shm.id'):
+        if start_guid:
             self.start_guid(verbose=verbose, notify_function=notify_function,
                             extra_guid_args=extra_guid_args)