XXX kvm: Implement external-disk

Support absolute and relative paths. Paths starting with "rbd:" are special,
and they point to the rbd endpoint, served by Ceph.

cache parameter is optional, in case if present, will be used as-is, even with
empty cache.

XXX:
 * improve
 * support cluster
 * support resilient?
 * find a way to test the rbd: magic
 * provide backward compatibility