sheep: pad a hole of opcodes

Cc: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
dog: judge vdi is standalone or not by vdi family graph

SD_MAX_CHILDREN is removed in the future because the new GC algorithm
resolves the limitation of a maximum number of children. So "dog alter
copy-vdi" command shouldn't depend on it. This patch lets the command
judge VDI is standalone or not by vdi family graph like "dog vdi
tree".

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