vdpa/mlx5: Add support for reading descriptor statistics

Implement the get_vq_stats calback of vdpa_config_ops to return the
statistics for a virtqueue.

The statistics are provided as vendor specific statistics where the
driver provides a pair of attribute name and attribute value.

Currently supported are received descriptors and completed descriptors.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Message-Id: <20220518133804.1075129-6-elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>