iotests.py: Let wait_migration() return on failure

iotests.py: Let wait_migration() return on failure

Let wait_migration() return on failure (with the return value indicating
whether the migration was completed or has failed), so we can use it for
migrations that are expected to fail, too.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200820150725.68687-3-mreitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>