sheep/dog: move opcode SD_OP_READ_DEL_VDI from sheepdog proto to internal proto

SD_OP_READ_DEL_VDI is used only dog&sheep communication.
so move definition SD_OP_READ_DEL_VDI from sheepdog_proto.h to internal_proto.h

[Hitoshi: modified commit log for correct description]

Signed-off-by Masaki Saeki <saeki.masaki@po.ntts.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>