To allow backups work across external snapshots we need to improve the
algorithm which calculates which bitmaps to merge.

The algorithm must look for appropriately named bitmaps in the image and
possibly descend into a backing image if the current image does not have
the bitmap.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>