Update table only when manager is visible (safe on cpu)

qubesmanager/main.py

@@ -532,6 +532,7 @@ class VmManagerWindow(QMainWindow):
     # When calling update_table() directly, always use out_of_schedule=True!
     def update_table(self, out_of_schedule=False):
 
+        if manager_window.isVisible():
             some_vms_have_changed_power_state = False
             for vm in self.vms_list:
                 state = vm.is_running();