virtio_pci_modern: hide vp_modern_get_queue_notify_off()

All users (both virtio-pci library and vp_vdpa driver) has been
switched to use vp_modern_map_vq_notify(). So there's no need to
export the low level helper of vp_modern_get_queue_notify_off().

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210415073147.19331-5-jasowang@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>