vdpa: factor out vhost_vdpa_pa_map() and vhost_vdpa_pa_unmap()

The upcoming patch is going to support VA mapping/unmapping.
So let's factor out the logic of PA mapping/unmapping firstly
to make the code more readable.

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/20210831103634.33-10-xieyongji@bytedance.com


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