dog: let "dog vdi cache" subcommands parse arguments in a correct manner

Current subcommands of "dog vdi cache" can cause segmentation faults
because they don't check an index of argv.

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

Conflicts:
	dog/vdi.c
Conflicts were resolved by Hitoshi Mitake.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>