Nested Hyper-V/Windows guest running on top of KVM will use TSC page
clocksource in two cases:
- L0 exposes invariant TSC (CPUID.80000007H:EDX[8]).
- L0 provides Hyper-V Reenlightenment support (CPUID.40000003H:EAX[13]).

Exposing invariant TSC effectively blocks migration to hosts with different
TSC frequencies, providing reenlightenment support will be needed when we
start migrating nested workloads.

Implement rudimentary support for reenlightenment MSRs. For now, these are
just read/write MSRs with no effect.
Signed-off-by: NVitaly Kuznetsov <vkuznets@redhat.com>
Reviewed-by: NRoman Kagan <rkagan@virtuozzo.com>
Signed-off-by: NRadim Krčmář <rkrcmar@redhat.com>