details: Don't traceback if VM deleted with open details window

When a VM is undefined, whether explicitly through virt-manager or behind
our back, we close and cleanup any associated vmmDetails windows. However
there's a race here: if the details window is the last remaining top
level window, cleanup() is called twice which tracebacks.

Fix this by dispatching the exit_app routine in an idle callback, to
avoid racing with any window delete routines.

Reported-by: Leonardo Garcia <lagarcia@br.ibm.com>

virtManager/engine.py

@@ -331,7 +331,9 @@ class vmmEngine(vmmGObject):
         if (self.windows <= 0 and
             self.systray and
             not self.systray.is_visible()):
-            self.exit_app(src)
+            # Defer this to an idle callback, since we can race with
+            # a vmmDetails window being deleted.
+            self.idle_add(self.exit_app, src)
 
     def _cleanup(self):
         uihelpers.cleanup()