tests: pass an new styled option of debug level to sheep

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