iotests: test clearing unknown autoclear_features by qcow2

Test clearing unknown autoclear_features by qcow2 on incoming
migration.

[ kwolf: Fixed wait for destination VM startup ]

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>