iotests: Use event_wait in wait_ready

Only poll the specific type of event we are interested in, to avoid
stealing events that should be consumed by someone else.

Suggested-by: John Snow <jsnow@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>