kvm/clock: add support for confidential guest reset

Confidential guests change the KVM VM file descriptor upon reset and also create
new VCPU file descriptors against the new KVM VM file descriptor. We need to
save the clock state from kvm before KVM VM file descriptor changes and restore
it after. Also after VCPU file descriptors changed, we must call
KVM_KVMCLOCK_CTRL on the VCPU file descriptor to inform KVM that the VCPU is
in paused state.

Signed-off-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/20260225035000.385950-32-anisinha@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>