x86, clock: Fix kvm guest tsc initialization

When booting a kvm guest on AMD with the latest kernel the following
messages are displayed in the boot log:

 tsc: Unable to calibrate against PIT
 tsc: HPET/PMTIMER calibration failed

aa297292 ("x86/tsc: Enumerate SKL cpu_khz and tsc_khz via CPUID")
introduced a change to account for a difference in cpu and tsc frequencies for
Intel SKL processors. Before this change the native tsc set
x86_platform.calibrate_tsc to native_calibrate_tsc() which is a hardware
calibration of the tsc, and in tsc_init() executed

	tsc_khz = x86_platform.calibrate_tsc();
	cpu_khz = tsc_khz;

The kvm code changed x86_platform.calibrate_tsc to kvm_get_tsc_khz() and
executed the same tsc_init() function.  This meant that KVM guests did not
execute the native hardware calibration function.

After aa297292, there are separate native calibrations for cpu_khz and
tsc_khz.  The code sets x86_platform.calibrate_tsc to native_calibrate_tsc()
which is now an Intel specific calibration function, and
x86_platform.calibrate_cpu to native_calibrate_cpu() which is the "old"
native_calibrate_tsc() function (ie, the native hardware calibration
function).

tsc_init() now does

	cpu_khz = x86_platform.calibrate_cpu();
	tsc_khz = x86_platform.calibrate_tsc();
	if (tsc_khz == 0)
		tsc_khz = cpu_khz;
	else if (abs(cpu_khz - tsc_khz) * 10 > tsc_khz)
		cpu_khz = tsc_khz;

The kvm code should not call the hardware initialization in
native_calibrate_cpu(), as it isn't applicable for kvm and it didn't do that
prior to aa297292.

This patch resolves this issue by setting x86_platform.calibrate_cpu to
kvm_get_tsc_khz().

v2: I had originally set x86_platform.calibrate_cpu to
cpu_khz_from_cpuid(), however, pbonzini pointed out that the CPUID leaf
in that function is not available in KVM.  I have changed the function
pointer to kvm_get_tsc_khz().

Fixes: aa297292 ("x86/tsc: Enumerate SKL cpu_khz and tsc_khz via CPUID")
Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: Len Brown <len.brown@intel.com>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: "Christopher S. Hall" <christopher.s.hall@intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: kvm@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>