vdpa/mlx5: add hardware queue moderation

vdpa/mlx5: add hardware queue moderation

The next parameters control the HW queue moderation feature.
This feature helps to control the traffic performance and latency
trade-off.

Each packet completion report from HW to SW requires CQ processing by SW
and triggers interrupt for the guest driver. Interrupt report and
handling cost CPU cycles and time and the amount of this affects
directly on packet performance and latency.

hw_latency_mode parameters [int]
  0, HW default.
  1, Latency is counted from the first packet completion report.
  2, Latency is counted from the last packet completion.
hw_max_latency_us parameters [int]
  0 - 4095, The maximum time in microseconds that packet completion
  report can be delayed.
hw_max_pending_comp parameter [int]
  0 - 65535, The maximum number of pending packets completions in an HW
queue.

Signed-off-by: Xueming Li <xuemingl@nvidia.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>