Code review comment for lp://qastaging/~jakedahn/nova/lp829687

Unfortunately this is not defined in the spec, so it needs to be registered as an extension. I know it feels ridiculous, but we need to be able to identify when our responses differ from the spec. All you need to do is add an ExtensionDescriptor in contrib. You will also need to update tests to expect the new value you are returning. May I also ask why call this 'vcpu' rather than 'vcpus' or 'cores'?