iotests: 055: Don't attach the target image already for drive-backup

Double attach is not a valid usage of the target image, drive-backup
will open the blockdev itself so skip the add_drive call in this case.

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