virtio-net: support device stats

This patch allows the driver to obtain some statistics from the device.

In the device implementation, we can count a lot of such information,
which can be used for debugging and judging the running status of the
device. We hope to directly display it to the user through ethtool.

To get stats atomically, try to get stats for all/multiple queue pairs
in one command.

Fixes: #180

Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>