sheep: rework get_vdis_from to reduce network overhead

Current get_vdis_from function prepare a buffer large enough (4M)
to get vdi state. It is not elegant and transfers too much between
sheep nodes.

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