virtio_net: reorder some funcs

The purpose of this is to facilitate the subsequent addition of new
functions without introducing a separate declaration.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>