KVM: x86: Add KVM_VCPU_TSC_SCALE and fix the documentation on TSC migration

KVM: x86: Add KVM_VCPU_TSC_SCALE and fix the documentation on TSC migration

The documentation on TSC migration using KVM_VCPU_TSC_OFFSET is woefully
inadequate. It ignores TSC scaling, and ignores the fact that the host
TSC may differ from one host to the next (and in fact because of the way
the kernel calibrates it, it generally differs from one boot to the next
even on the same hardware).

Add KVM_VCPU_TSC_SCALE to extract the actual scale ratio and frac_bits,
and attempt to document the process that userspace needs to follow to
preserve the TSC across migration.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>