net: virtio: switch to netif_napi_add_weight()

virtio netdev driver uses a custom napi weight, switch to the new
API for setting custom weight.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>