Introduce a new parameter for qemu-img commit which may be used to
explicitly specify the backing file into which an image should be
committed if the backing chain has more than a single layer.

[Applied Eric Blake's qemu-img.texi documentation rewording
--Stefan]
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 1414159063-25977-12-git-send-email-mreitz@redhat.com
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>