KVM: x86: Avoid global clock update on setting KVM clock MSR

KVM: x86: Avoid global clock update on setting KVM clock MSR

Commit 0061d53daf26 ("KVM: x86: limit difference between kvmclock updates")
introduced a KVM_REQ_GLOBAL_CLOCK_UPDATE when one vCPU set up its clock.

This was a workaround for the ever-drifting clocks which were based on the
host's CLOCK_MONOTONIC and thus subject to NTP skew. On booting or resuming
a guest, it just leads to running kvm_guest_time_update() twice for each
vCPU for now good reason.

Just use KVM_REQ_CLOCK_UPDATE on the vCPU itself, and only in the case
where the KVM clock is being set up, not turned off.

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