iotests: Make 234 stable

iotests: Make 234 stable

This test waits for a MIGRATION event with status=completed on the
source VM before querying the migration status on both source and
destination.  However, just because the source says migration has
completed does not mean the destination thinks the same.  Therefore, in
some cases, the destination VM may still report "active" instead of
"completed" when asked for its migration status.

Fix this by enabling migration events on both VMs and waiting until both
source and destination emit a status=completed MIGRATION event.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>