KVM: x86: Kill KVM_REQ_GLOBAL_CLOCK_UPDATE

KVM: x86: Kill KVM_REQ_GLOBAL_CLOCK_UPDATE

This was introduced in commit 0061d53daf26 ("KVM: x86: limit difference
between kvmclock updates") to reduce cross-vCPU differences which arose
because the KVM clock was based on CLOCK_MONOTONIC and thus subject to
NTP frequency corrections.

However, commit 53fafdbb8b21 ("KVM: x86: switch KVMCLOCK base to
monotonic raw clock") switched to using CLOCK_MONOTONIC_RAW as the basis
for the KVM clock, avoiding the NTP frequency skew altogether.

So remove KVM_REQ_GLOBAL_CLOCK_UPDATE. In kvm_write_system_time(), all
that's needed is a single KVM_REQ_CLOCK_UPDATE for the vCPU whose KVM
clock is being configured.

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