vduse: make vduse_class constant

Now that the driver core allows for struct class to be in read-only
memory, we should make all 'class' structures declared at build time
placing them into read-only memory, instead of having to be dynamically
allocated at runtime.

Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Cc: Xie Yongji <xieyongji@bytedance.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Message-Id: <2023100643-tricolor-citizen-6c2d@gregkh>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>