vduse: Support registering userspace memory for IOVA regions

Introduce two ioctls: VDUSE_IOTLB_REG_UMEM and
VDUSE_IOTLB_DEREG_UMEM to support registering
and de-registering userspace memory for IOVA
regions.

Now it only supports registering userspace memory
for bounce buffer region in virtio-vdpa case.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20220803045523.23851-5-xieyongji@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>