The backing format should be honored during image creation.  For some
reason we currently use the image format to open the backing file.  This
fails when the backing file has a different format than the image being
created.  Keep the image and backing format drivers completely separate.

Also print the backing filename if there is an error opening the backing
file instead of the image filename.
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Acked-by: NJes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>