There are at this time about a bazillion comments here pointing out that a privilege escalation that requires root access is not a privilege escalation.
I don't know what the authors of those comments were doing for the past 5 years, because they should really consider whether they are qualified to talk on the subject. AMD and Intel have been incorporating virtualization and paravirtualization support into their CPUs for a long time, and there is a massive market for these solutions. For an equal amount of time security researchers have been messing around finding exploits like this one in the hardware. Privilege escalation from domain to hypervisor/cross-domain level is a breach of the virtualization security model, and you can bet your ass it's a serious security issue. And if your favorite virtualization solution doesn't consider this a root exploit, then that solution is broken. Because there's no way anyone in their right mind running something like 50 domains on some 24-core beast - made specifically to virtualize the crap out of everything - will consider those domains being able to get root in all other domains to be anything short of a huge problem.
tl;dr: root is not root if you are in a guest domain. (cue inane Matrix reference to taste)