vDPA/ifcvf: implement vdpa_config_ops.get_vq_size

This commit implements vdpa_ops.get_vq_size to report
the size of a specific virtqueue.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20240202163905.8834-4-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>