QEMU 0.8.2 allows local users to halt a virtual machine by executing the icebp instruction.
https://exchange.xforce.ibmcloud.com/vulnerabilities/34043
http://www.vupen.com/english/advisories/2007/1597
http://www.securityfocus.com/bid/23731
http://www.mandriva.com/security/advisories?name=MDVSA-2008:162
http://www.debian.org/security/2007/dsa-1284
http://taviso.decsystem.org/virtsec.pdf
http://secunia.com/advisories/29129