events: implicitly enable caching when creating EventsDispatcher

Application that runs EventsDispatcher can safely use also cache , which
greatly improve performance. This is because cache then is properly
updated/invalidated when needed.
Instead of modifying each application to explicitly enable cache based
on this simple rule, make it implicit when EventsDispatcher is created.

QubesOS/qubes-issues#3293

qubesadmin/events/__init__.py

@@ -32,9 +32,17 @@ class EventsDispatcher(object):
     ''' Events dispatcher, responsible for receiving events and calling
     appropriate handlers'''
     def __init__(self, app, api_method='admin.Events'):
-        '''Initialize EventsDispatcher'''
+        """Initialize EventsDispatcher
+
+        Connecting :py:class:`EventsDispatcher` object to a
+        :py:class:`qubesadmin.Qubes` implicitly enables caching. It is important
+        to actually run the dispatcher (:py:meth:`listen_for_events`), otherwise
+        the cache won't be updated. Alternatively, disable caching by setting
+        :py:attr:`qubesadmin.Qubes.cache_enabled` property to `False`.
+        """
         #: Qubes() object
         self.app = app
+        self.app.cache_enabled = True
 
         self._api_method = api_method