sheep: don't update vdi_iocb in vdi_* functions

We are using vdi_iocb to get a snapid from a vdi object, but it looks
like abusing of vdi_iocb.  This patch makes how to use vdi_iocb and
vdi_info clear like as follows.

 - vdi_iocb is used to pass parameters from clients.
 - vdi_info is used to get info from sheepdog.

To prevent from abusing them again, this patch makes some vdi_iocb
variables constant where possible.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>