python/aqmp: add .empty() method to EventListener

python/aqmp: add .empty() method to EventListener

Synchronous clients may want to know if they're about to block waiting
for an event or not. A method such as this is necessary to implement a
compatible interface for the old QEMUMonitorProtocol using the new async
internals.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 20210923004938.3999963-3-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>