dog: revive verbose option

Some subcommands of "dog vdi" prints VID of newly created VDI when -v
(verbose) option is passed. But the option seems to be dead because of
an invalid error handling. This patch revives it.

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