KVM: x86: Override TSC_STABLE flag for Xen PV clocks in kvm_guest_time_update()

When updating PV clocks, handle the Xen-specific UNSTABLE_TSC override in
the main kvm_guest_time_update() by simply clearing PVCLOCK_TSC_STABLE_BIT
in the flags of the reference pvclock structure.  Expand the comment to
(hopefully) make it obvious that Xen clocks need to be processed after all
clocks that care about the TSC_STABLE flag.

No functional change intended.

Cc: Paul Durrant <pdurrant@amazon.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20250201013827.680235-12-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>