KVM: x86/xen: Use guest's copy of pvclock when starting timer

KVM: x86/xen: Use guest's copy of pvclock when starting timer

Use the guest's copy of its pvclock when starting a Xen timer, as KVM's
reference copy may not be up-to-date, i.e. may yield a false positive of
sorts.  In the unlikely scenario that the guest is starting a Xen timer
and has used a Xen pvclock in the past, but has since but turned it "off",
then vcpu->arch.hv_clock may be stale, as KVM's reference copy is updated
if and only if at least one pvclock is enabled.

Furthermore, vcpu->arch.hv_clock is currently used by three different
pvclocks: kvmclock, Xen, and Xen compat.  While it's extremely unlikely a
guest would ever enable multiple pvclocks, effectively sharing KVM's
reference clock could yield very weird behavior.  Using the guest's active
Xen pvclock instead of KVM's reference will allow dropping KVM's
reference copy.

Fixes: 451a707813ae ("KVM: x86/xen: improve accuracy of Xen timers")
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/20250201013827.680235-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>