dog: fix a core dump by 'vdi lock force-unlock'

force-unlock expect an argument so we need to set CMD_NEED_ARG for it otherwise
a core will be generated by

$ dog vdi lock force-unlock # without argument

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
dog: rename 'force-unlock' as 'unlock'

We don't have a command named 'vdi lock unlock', so we can simply take this seat
and simplify the command line for users.

updated:
$ dog vdi lock unlock <vdiname>

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