KVM: x86: Remove implicit rdtsc() from kvm_compute_l1_tsc_offset()

KVM: x86: Remove implicit rdtsc() from kvm_compute_l1_tsc_offset()

Let the callers pass the host TSC value in as an explicit parameter.

This leaves some fairly obviously stupid code, which using this function
to compare the guest TSC at some *other* time, with the newly-minted TSC
value from rdtsc(). Unless it's being used to measure *elapsed* time,
that isn't very sensible.

In this case, "obviously stupid" is an improvement over being non-obviously
so.

No functional change intended.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>