sheep: don't recycle VDI ID

Recycling VDI IDs of deleted VDIs is a completely wrong idea. It
breaks relations between inode objects and data objects. For example,
it can cause a problem of corrupting cloned VDIs (see related
issue). This patch forbids the recycling.

Related issue:
https://bugs.launchpad.net/sheepdog-project/+bug/1317755

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