block: Accept any target node for transactional blockdev-backup

block: Accept any target node for transactional blockdev-backup

Commit 0d978913 changed blockdev-backup to accept arbitrary node names
instead of device names (i.e. root nodes) for the backup target.
However, it forgot to make the same change in transactions and to update
the documentation. This patch fixes these omissions.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>