vdpa/mlx5: Add support for doorbell bypassing

Implement mlx5_get_vq_notification() to return the doorbell address.
Since the notification area is mapped to userspace, make sure that the
BAR size is at least PAGE_SIZE large.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20210603081153.5750-1-elic@nvidia.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>