KVM: x86: Don't bleed PVCLOCK_GUEST_STOPPED across PV clocks

KVM: x86: Don't bleed PVCLOCK_GUEST_STOPPED across PV clocks

When updating a specific PV clock, make a full copy of KVM's reference
copy/cache so that PVCLOCK_GUEST_STOPPED doesn't bleed across clocks.
E.g. in the unlikely scenario the guest has enabled both kvmclock and Xen
PV clock, a dangling GUEST_STOPPED in kvmclock would bleed into Xen PV
clock.

Using a local copy of the pvclock structure also sets the stage for
eliminating the per-vCPU copy/cache (only the TSC frequency information
actually "needs" to be cached/persisted).

Fixes: aa096aa0a05f ("KVM: x86/xen: setup pvclock updates")
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20250201013827.680235-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>