sheepdog: adopting protocol update for VDI locking
The update is required for supporting iSCSI multipath. It doesn't
affect behavior of QEMU driver but adding a new field to vdi request
struct is required.
Cc: Kevin Wolf <kwolf@redhat.com> Cc: Stefan Hajnoczi <stefanha@redhat.com> Cc: Liu Yuan <namei.unix@gmail.com> Cc: MORITA Kazutaka <morita.kazutaka@gmail.com> Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>