qemu-iotests: Test blockdev-backup in 055

This applies cases on drive-backup on blockdev-backup, except cases with
target format and mode.

Also add a case to check source == target.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1418899027-8445-5-git-send-email-famz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
