vduse: implement vdpa_config_ops.get_vq_size for vduse

This commit implements get_vq_size for vdpa_config_ops. This
new interface is used to report per vq size.

Signed-off-by: Zhu Lingshan <lingshan.zhu@intel.com>
Message-Id: <20240202163905.8834-8-lingshan.zhu@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>