sheepdog: fix clone operation by 'qemu-img create -b'

We should pass base_inode->vdi_id to base_vdi_id of SheepdogVdiReq so that sheep
can create a clone instead a fresh volume.

This fixes following command:

qemu-create -b sheepdog:base sheepdog:clone

so users can boot sheepdog:clone as a normal volume.

Cc: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>