python/qmp: remove Server.wait_closed() call for Python 3.12

This patch is a backport from
https://gitlab.com/qemu-project/python-qemu-qmp/-/commit/e03a3334b6a477beb09b293708632f2c06fe9f61

According to Guido in python/cpython#104344 ,
this call was never meant to wait for the server to shut down - that is
handled synchronously - but instead, this waits for all connections to
close. Or, it would have, if it wasn't broken since it was introduced.

3.12 fixes the bug, which now causes a hang in our code. The fix is just
to remove the wait.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-id: 20231006195243.3131140-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/qmp/protocol.py

@@ -495,7 +495,6 @@ async def _stop_server(self) -> None:
         try:
             self.logger.debug("Stopping server.")
             self._server.close()
-            await self._server.wait_closed()
             self.logger.debug("Server stopped.")
         finally:
             self._server = None