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>
Merge branch 'stable-0.8' of github.com:sheepdog/sheepdog into stable-0.8