dog: add 'vdi object map' command

This command will show inode internal 'vdi' array states, is useful to check if
cloned VDI's inode is sane or not.

usage:
yliu@ubuntu-precise:~/sheepdog$ dog/dog vdi object map clone1
Index       VID
00000000   c8f989
00000001   c8f989
00000002   c8f989
00000003   7c2b25
00000004   7c2b25
00000005   7c2b25
...

yliu@ubuntu-precise:~/sheepdog$ dog/dog vdi object map -i 4 clone1
Index       VID
00000004   7c2b25

With this patch, original 'dog vdi object' becomes 'dog vdi object location'

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