sheep: correct error code of vdi deletion

The previous refactoring commit (5e460b4d64441) broke QA 016. This
patch fixes the problem.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: remove fd leak in vdi deletion process

The commit 5e460b4d6444 introduced fd leak in the vdi deletion
process. This patch removes it.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
sheep: cleaning vdi deletion process, round 2

This patch cleans the vdi deletion process of sheep. Current vdi
deletion code creates one work for one deletion. It seems that there
is no reason to create so many work. This patch lets the code to
create only one work for vdi deletion request.

With this change, this patch improves simplicity of the code. For
example, the queue of struct deletion_info and related code are
removed.

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

Conflicts:
	sheep/vdi.c

Conflicts were resolved by Hitoshi Mitake.
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>