vdpa/mlx5: optimize completion queue poll

vdpa/mlx5: optimize completion queue poll

The vDPA driver uses a CQ in order to know when traffic works were
completed by the HW.

Each traffic burst completion adds a CQE to the CQ.

When the vDPA driver detects CQEs in the CQ, it triggers the guest
notification for the corresponding queue and consumes all of them.

There is collapse feature in the HW that configures the HW to write all
the CQEs in the first entry of the CQ.

Using this feature, the vDPA driver can read only the first CQE,
validate that the completion counter inside the CQE was changed and if
so, to notify the guest.

Use CQ collapse feature in order to improve the poll utilization.

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