The solution for bz 1490784 blocked RHEL < 6.9 on PPC for compatibility level 4.2 (since they were not supported on pseries-rhel7.4.0+). Later they were blocked also for compatibility level 4.3 for the same reason.

The problem is that we need to manually add every new compatibility version to that list now (and of course we didn't since 4.4).

As we dropped compatibility levels <= 4.1, we can now just block them altogether - we can introduce a validation that rejects this OS type, and remove it from the list of operating systems on new/edit VM.