iotests: add QMP event waiting queue

A filter is added to allow callers to request very specific
events to be pulled from the event queue, while leaving undesired
events still in the stream.

This allows us to poll for completion data for multiple asynchronous
events in any arbitrary order.

A new timeout context is added to the qmp pull_event method's
wait parameter to allow tests to fail if they do not complete
within some expected period of time.

Also fixed is a bug in qmp.pull_event where we try to retrieve an event
from an empty list if we attempt to retrieve an event with wait=False
but no events have occurred.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1429314609-29776-19-git-send-email-jsnow@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>