Selinux should block the VENOM vulnerability in Qemu: http://danwalsh.livejournal.com/71489.html
See also http://www.selinuxproject.org/page/SVirt
Keep in mind that Grsec (which is definitely underdeployed in the linux community) is compatible with LSM such as SElinx and AppArmor, so you do not have to choose (you'll take a performance hit, though).