drive-mirror RBD thin
Description of problem:
I found that this problem was first discovered in 2014. There was a post 
[2014 bug description](https://lists.gnu.org/archive/html/qemu-devel/2014-10/msg01231.html )、
[2014 patch](https://patchwork.ozlabs.org/project/qemu-devel/patch/1433747185-16797-2-git-send-email-famz@redhat.com/)
mentioning this bug. 
The patch in the post said that this problem had been solved, but after trying and asking, I found that the problem had not been solved.
Later, I saw this problem in the [2017 bug description](https://forum.proxmox.com/threads/drive-mirror-rbd-thin.33250/#post-613502) forum and it was said that there was a patch to fix it, but it was not.
I tried the latest qemu version and found that this problem has not been solved.
Additional information:
nbd is normal, but rbd is wrong！