virtio-net: Constify VirtIOFeature feature_sizes[]

Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-Id: <20210511104157.2880306-4-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>