dog/vdi: enable to unlock a vdi locked by tgtd

Explicitly add vdi.type parameter in RELEASE_VDI request.

[Hitoshi: fixed trivial whitespace probelm]

Signed-off-by: YAMADA Hideki <yamada.hideki@lab.ntt.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>