sheepdog: fix 'qemu-img map'

It was muted in the previous commit 4bc74be9. Let's revive it since nothing
prevents us to do it.

With this patch, following command will work as other formats:

$ qemu-img map sheepdog:image

Cc: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
