python/aqmp: Return cleared events from EventListener.clear()

This serves two purposes:

(1) It is now possible to discern whether or not clear() removed any
event(s) from the queue with absolute certainty, and

(2) It is now very easy to get a List of all pending events in one
chunk, which is useful for the sync bridge.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20210923004938.3999963-4-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/aqmp/events.py

@@ -562,7 +562,7 @@ class EventListener:
         """
         return self._queue.empty()
 
-    def clear(self) -> None:
+    def clear(self) -> List[Message]:
         """
         Clear this listener of all pending events.
 
@@ -570,17 +570,22 @@ class EventListener:
         pending FIFO queue synchronously. It can be also be used to
         manually clear any pending events, if desired.
 
+        :return: The cleared events, if any.
+
         .. warning::
             Take care when discarding events. Cleared events will be
             silently tossed on the floor. All events that were ever
             accepted by this listener are visible in `history()`.
         """
+        events = []
         while True:
             try:
-                self._queue.get_nowait()
+                events.append(self._queue.get_nowait())
             except asyncio.QueueEmpty:
                 break
 
+        return events
+
     def __aiter__(self) -> AsyncIterator[Message]:
         return self