KVM: x86: add new KVMCLOCK cpuid feature

This cpuid, KVM_CPUID_CLOCKSOURCE2, will indicate to the guest
that kvmclock is available through a new set of MSRs. The old ones
are deprecated.

Signed-off-by: Glauber Costa <glommer@redhat.com>
Acked-by: Zachary Amsden <zamsden@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>