vhost-iotlb: Add an opaque pointer for vhost IOTLB

Add an opaque pointer for vhost IOTLB. And introduce
vhost_iotlb_add_range_ctx() to accept it.

Suggested-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210831103634.33-8-xieyongji@bytedance.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>