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>
Revert "sheep: remove invalid usage of current_vnode_info"

This reverts commit e2a8b0605e85fcae742738dd5ca518ea4ea3bd44.
Merge branch 'stable-0.7' of github.com:collie/sheepdog into stable-0.7