Ok, May I know why you think it's only a bug in SLES?. I never claimed it is a bug in SLES, but a bug in Ovirt detecting proper memory usage in SLES. The behaviour you observe was normal for RHEL6/CentOS6/SLES11/openSUSE and bellow , so it is normal for some OSes.In my oVirt 4.3.10 , I see that the entry there is "SLES11+" , but I believe that it is checking the memory on SLES15 , just as if it is a SLES11.
As I said before, ovirt is behaving the same way even for CentOS7 VMs. I am attaching the details again here below. Most probably oVirt is checking memory the RHEL6 style , which is not the correct one.
My question is why ovirt is treating buff/cache memory as used memory and why is not reporting memory usage just based on actual used memory? Most probably it is a bug :D , every software has some. I would recommend you to open a bug in the bugzilla.redhat.com for each OS type (for example 1 for SLES/openSUSE and 1 for EL7/EL8-based).
Best Regards, Strahil Nikolov