virtio_net: Use virtio_find_vqs_ctx() helper

virtio_find_vqs_ctx() is defined but never be called currently,
it is the right place to use it.

Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>