block: Allow backing file links in change_parent_backing_link()

Now that the backing file child role implements .attach/.detach
callbacks, nothing prevents us from modifying the graph even if that
involves changing backing file links.

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