When running migration tests we monitor for a STOP event so we can skip
redundant waits. This will be needed for the RESUME event too shortly.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20230601161347.1803440-8-berrange@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>