As other people mentioned, it seems more likely that they're running a Xen fork ...

jsabo on May 13, 2015 | parent | context | favorite | on: Venom – A security vulnerability in virtual floppy...

As other people mentioned, it seems more likely that they're running a Xen fork or have some other mitigation. The redhat notes state that once the host is patched, the VMs need to be shut down for it to take effect: https://rhn.redhat.com/errata/RHSA-2015-0998.html