vdpa/mlx5: support virtio queue statistics get

Add support for statistics operations.

A DevX counter object is allocated per virtq in order to
manage the virtq statistics.

The counter object is allocated before the virtq creation
and destroyed after it, so the statistics are valid only in
the life time of the virtq.

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