virtio_net: support device stats

As the spec oasis-tcs/virtio-spec@42f3899

make virtio-net support getting the stats from the device by ethtool -S
<eth0>.

NIC statistics:
     rx0_packets: 582951
     rx0_bytes: 155307077
     rx0_drops: 0
     rx0_xdp_packets: 0
     rx0_xdp_tx: 0
     rx0_xdp_redirects: 0
     rx0_xdp_drops: 0
     rx0_kicks: 17007
     rx0_hw_packets: 2179409
     rx0_hw_bytes: 510015040
     rx0_hw_notifications: 0
     rx0_hw_interrupts: 0
     rx0_hw_needs_csum: 2179409
     rx0_hw_ratelimit_bytes: 0
     tx0_packets: 15361
     tx0_bytes: 1918970
     tx0_xdp_tx: 0
     tx0_xdp_tx_drops: 0
     tx0_kicks: 15361
     tx0_timeouts: 0
     tx0_hw_packets: 32272
     tx0_hw_bytes: 4311698
     tx0_hw_notifications: 0
     tx0_hw_interrupts: 0
     tx0_hw_ratelimit_bytes: 0

The follow stats are hidden, there are exported by the queue stat API
in the subsequent comment.

    VIRTNET_STATS_DESC_RX(basic, drops)
    VIRTNET_STATS_DESC_RX(basic, drop_overruns),
    VIRTNET_STATS_DESC_TX(basic, drops),
    VIRTNET_STATS_DESC_TX(basic, drop_malformed),
    VIRTNET_STATS_DESC_RX(csum, csum_valid),
    VIRTNET_STATS_DESC_RX(csum, csum_none),
    VIRTNET_STATS_DESC_RX(csum, csum_bad),
    VIRTNET_STATS_DESC_TX(csum, needs_csum),
    VIRTNET_STATS_DESC_TX(csum, csum_none),
    VIRTNET_STATS_DESC_RX(gso, gso_packets),
    VIRTNET_STATS_DESC_RX(gso, gso_bytes),
    VIRTNET_STATS_DESC_RX(gso, gso_packets_coalesced),
    VIRTNET_STATS_DESC_RX(gso, gso_bytes_coalesced),
    VIRTNET_STATS_DESC_TX(gso, gso_packets),
    VIRTNET_STATS_DESC_TX(gso, gso_bytes),
    VIRTNET_STATS_DESC_TX(gso, gso_segments),
    VIRTNET_STATS_DESC_TX(gso, gso_segments_bytes),
    VIRTNET_STATS_DESC_RX(speed, ratelimit_packets),
    VIRTNET_STATS_DESC_TX(speed, ratelimit_packets),

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