vdpa: introduce get_vq_notification method

This patch introduces a new method in the vdpa_config_ops which
reports the physical address and the size of the doorbell for a
specific virtqueue.

This will be used by the future patches that maps doorbell to
userspace.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20200529080303.15449-4-jasowang@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>