virtio-net: support queue stat

To enhance functionality, we now support reporting statistics through
the netdev-generic netlink (netdev-genl) queue stats interface. However,
this does not extend to all statistics, so a new field, qstat_offset,
has been introduced. This field determines which statistics should be
reported via netdev-genl queue stats.

Given that queue stats are retrieved individually per queue, it's
necessary for the virtnet_get_hw_stats() function to be capable of
fetching statistics for a specific queue.

As the document https://docs.kernel.org/next/networking/statistics.html#notes-for-driver-authors

We should not duplicate the stats which get reported via the netlink API in
ethtool. If the stats are for queue stat, that will not be reported by
ethtool -S.

python3 ./tools/net/ynl/cli.py --spec Documentation/netlink/specs/netdev.yaml
    --dump qstats-get --json '{"scope": "queue"}'
[{'ifindex': 2,
  'queue-id': 0,
  'queue-type': 'rx',
  'rx-bytes': 157844011,
  'rx-csum-bad': 0,
  'rx-csum-none': 0,
  'rx-csum-unnecessary': 2195386,
  'rx-hw-drop-overruns': 0,
  'rx-hw-drop-ratelimits': 0,
  'rx-hw-drops': 12964,
  'rx-packets': 598929},
 {'ifindex': 2,
  'queue-id': 0,
  'queue-type': 'tx',
  'tx-bytes': 1938511,
  'tx-csum-none': 0,
  'tx-hw-drop-errors': 0,
  'tx-hw-drop-ratelimits': 0,
  'tx-hw-drops': 0,
  'tx-needs-csum': 61263,
  'tx-packets': 15515}]
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>