Avoid resetting clocksync service of just enabled clockvm

When setting global clockvm property, 'clocksync' service is
automatically added to the new value and then removed from the old one.
But if the new value is the same as the old one, the service gets
removed from the just set new value.
Check for this case explicitly.

Fixes QubesOS/qubes-issues#4939

qubes/app.py

@@ -1473,6 +1473,8 @@ class Qubes(qubes.PropertyHolder):
     @qubes.events.handler('property-set:clockvm')
     def on_property_set_clockvm(self, event, name, newvalue, oldvalue=None):
         # pylint: disable=unused-argument,no-self-use
+        if oldvalue == newvalue:
+            return
         if oldvalue and oldvalue.features.get('service.clocksync', False):
             del oldvalue.features['service.clocksync']