Do not try reading "old" property value when loading them from XML

When loading properties from XML, events are disabled. Do not try to
retrieve old property value that would be used only for sending an
event. Due to dependencies between properties, it may not be possible
yet.

Theoretically, some of such issues could be solved by shuffling
properties between load stages, but there are many layers of inter VM
dependencies (TemplateVM->AppVM, netvm, guivm, default_dispvm etc) and
solving them all at once might not be possible in some configurations.

Fixes QubesOS/qubes-issues#6998

(cherry picked from commit b6a0b40)

qubes/__init__.py

@@ -245,8 +245,11 @@ def __set__(self, instance, value):
             return
 
         try:
-            oldvalue = getattr(instance, self.__name__)
-            has_oldvalue = True
+            if instance.events_enabled:
+                oldvalue = getattr(instance, self.__name__)
+                has_oldvalue = True
+            else:
+                has_oldvalue = False
         except AttributeError:
             has_oldvalue = False