virt-manager: Avoid segfault on unset DISPLAY

Gtk.Window() init was crashing the app. Instead use Gtk.init() for
testing that things are working okay

virt-manager

@@ -80,7 +80,7 @@ def _import_gtk(leftovers):
             sys.exit(1)
 
         # This will error if Gtk wasn't correctly initialized
-        Gtk.Window()
+        Gtk.init()
 
         globals()["Gtk"] = Gtk
         import virtManager.config