vhost: introduce operation to get vDPA queue stats

vhost: introduce operation to get vDPA queue stats

The vDPA device offloads all the datapath of the vhost
device to the HW device.

In order to expose to the user traffic information this
patch introduces new 3 APIs to get traffic statistics, the
device statistics name and to reset the statistics per
virtio queue.

The statistics are taken directly from the vDPA driver
managing the HW device and can be different for each vendor
driver.

Signed-off-by: Matan Azrad <matan@mellanox.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>