storage: Implement backing store support for "fat:" prefix

storage: Implement backing store support for "fat:" prefix

qemublocktest showed that we don't add the "fat:" prefix for directory
storage when formatting the backing store string. While it's unlikely to
be used it's simple enough to actually implement the support rather than
trying to forbid it.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>