Since we're changing the msrs kvmclock uses, we have to communicate
that to the guest, through cpuid. We can add a new KVM_CAP to the
hypervisor, and then patch userspace to recognize it.
And if we ever add a new cpuid bit in the future, we have to do that again,
which create some complexity and delay in feature adoption.
Instead, what I'm proposing in this patch is a new capability, called
KVM_CAP_X86_CPUID_FEATURE_LIST, that returns the current feature list
currently supported by the hypervisor. If we ever want to add or remove
some feature, we only need to tweak into the HV, leaving userspace untouched.