KVM guests must not be allowed to update the host CPU microcode. KVM does not allows this and instead always returns the same microcode revision or patch level value to the guest. If the guest tries to update the CPU microcode, it will fail and show an error message similar to:
CPU0: update failed (for patch_level=0x6000624)
To work around this, configure the guest to not install CPU microcode updates; for example, uninstall the microcode_ctl package Red Hat Enterprise Linux of Fedora guests.