vdpa: Add an opaque pointer for vdpa_config_ops.dma_map()

Add an opaque pointer for DMA mapping.

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/20210831103634.33-9-xieyongji@bytedance.com


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