vhost-vdpa: uAPI to get virtqueue group id

Follows the support for virtqueue group in vDPA. This patches
introduces uAPI to get the virtqueue group ID for a specific virtqueue
in vhost-vdpa.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Gautam Dawar <gdawar@xilinx.com>
Message-Id: <20220330180436.24644-14-gdawar@xilinx.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>