Don't lookup current monitor

At the point we populate the menus, the window is not mapped,
so we don't know what monitor it will be on. The
gtk_widget_get_window() call will return NULL.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
2 jobs for master in 2 minutes and 23 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #145088536
jobfedora29

00:02:23

passed #145088537
jobubuntu1804

00:01:44