pds_vdpa: virtio bar setup for vdpa

Prep and use the "modern" virtio bar utilities to get our
virtio config space ready.

Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230519215632.12343-8-shannon.nelson@amd.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>