sheep: cleaning vdi deletion process of sheep daemon

Current code of vdi deletion process in sheep deamon is a real
disaster. Especially, its irregular usage of work queue is terrible
thing for maintenance. In addition, the vdi deletion process can
potentially notify a completion of deletion to a dog process before
actual completion.

This patch cleans the VDI deletion code and removes the above bug.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>