virtio: remove export for virtio_config_{enable, disable}

virtio_config_enable(), virtio_config_disable() are only used inside
drivers/virtio/virtio.c, so it doesn't need export the symbols.

Signed-off-by: Xianting Tian <xianting_tian@126.com>
Link: https://lore.kernel.org/r/1613838498-8791-1-git-send-email-xianting_tian@126.com


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