events: ignore events from non-existing domains

It may happen that when client handle the event, domain no longer
exists. This is for example common for DispVMs, which get removed just
after shutdown.
This will cause some events to be dropped, but one can enable blind
mode, to get them anyway (because it will not cause KeyError, even if
domain is already removed).

QubesOS/qubes-issues#3100

qubesadmin/events/__init__.py

@@ -191,7 +191,10 @@ class EventsDispatcher(object):
         if subject:
             if event in ['property-set:name']:
                 self.app.domains.clear_cache()
-            subject = self.app.domains[subject]
+            try:
+                subject = self.app.domains[subject]
+            except KeyError:
+                return
         else:
             # handle cache refreshing on best-effort basis
             if event in ['domain-add', 'domain-delete']: