KVM: hyperv: support HV_X64_MSR_TSC_FREQUENCY and HV_X64_MSR_APIC_FREQUENCY

It has been experimentally confirmed that supporting these two MSRs is one
of the necessary conditions for nested Hyper-V to use the TSC page. Modern
Windows guests are noticeably slower when they fall back to reading
timestamps from the HV_X64_MSR_TIME_REF_COUNT MSR instead of using the TSC
page.

The newly supported MSRs are advertised with the AccessFrequencyRegs
partition privilege flag and CPUID.40000003H:EDX[8] "Support for
determining timer frequencies is available" (both outside of the scope of
this KVM patch).

Reviewed-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>