tests/functional: update 041 for a case of restoring to non-existing snapshot

dog must report an error in such a case.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
dog: check existence of specified snapshot in vdi_restore()

Current "dog vdi restore" doesn't check existence of specified
snapshot. So if users specify invalid snapshot ID or tag, the parent
VDI is simply deleted. This behavior causes serious problem. This
patch adds the existence check for preventing it.

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