Since the debian bug has been marked as fixed and there wasn't any response to the question in my last comment, I assume this has been fixed in upstream QEMU, too. So closing this ticket accordingly...
** Changed in: qemu Status: Incomplete => Fix Released -- You received this bug notification because you are a member of qemu- devel-ml, which is subscribed to QEMU. https://bugs.launchpad.net/bugs/899961 Title: qemu/kvm locks up when run 32bit userspace with 64bit kernel Status in QEMU: Fix Released Status in qemu-kvm package in Debian: Fix Released Bug description: Only applies to qemu-kvm 1.0, and only when kernel is 64bit and userspace is 32bit, on x86. Did not happen with previous released versions, such as 0.15. Not all guests triggers this issue - so far, only (32bit) windows 7 guest shows it, but does that quite reliable: first boot of an old guest with new qemu-kvm, windows finds a new CPU and suggests rebooting - hit "Reboot" and in a few seconds it will be locked up (including the monitor), with no CPU usage whatsoever. Killable only with -9. To manage notifications about this bug go to: https://bugs.launchpad.net/qemu/+bug/899961/+subscriptions