KVM guest: disable stealtime on reboot to avoid mem corruption

else, host continues to update stealtime after reboot,
which can corrupt e.g. initramfs area.
found when tracking down initramfs unpack error on initial reboot
(with qemu-kvm -smp 2, no problem with single-core).

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>