libqtest: add qmp_async

Add qmp_async, which lets us send QMP commands asynchronously.
This is useful when we want to send commands that will trigger
event responses, but we don't know in what order to expect them.

Sometimes the event responses may arrive even before the command
confirmation will show up, so it is convenient to leave the responses
in the stream.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 1426018503-821-5-git-send-email-jsnow@redhat.com