i386/kvm: advertise Hyper-V frequency MSRs

As of kernel commit eb82feea59d6 ("KVM: hyperv: support HV_X64_MSR_TSC_FREQUENCY
and HV_X64_MSR_APIC_FREQUENCY"), KVM supports two new MSRs which are required
for nested Hyper-V to read timestamps with RDTSC + TSC page.

This commit makes QEMU advertise the MSRs with CPUID.40000003H:EAX[11] and
CPUID.40000003H:EDX[8] as specified in the Hyper-V TLFS and experimentally
verified on a Hyper-V host. The feature is enabled with the existing hv-time CPU
flag, and only if the TSC frequency is stable across migrations and known.

Signed-off-by: Ladi Prosek <lprosek@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Message-Id: <20170807085703.32267-5-lprosek@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>