sheepdog: allow to delete snapshot

This patch implements a blockdriver function bdrv_snapshot_delete() in
the sheepdog driver. With the new function, snapshots of sheepdog can
be deleted from libvirt.

Cc: Jeff Cody <jcody@redhat.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>
Message-id: 1450873346-22334-1-git-send-email-mitake.hitoshi@lab.ntt.co.jp
Signed-off-by: Jeff Cody <jcody@redhat.com>