virtio_net: device stats helpers support driver stats

In the last commit, we introduced some helpers for device stats.
And the drivers stats are realized by the open code.
This commit make the helpers to support driver stats.
Then we can have the unify helper for device and driver stats.

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>