After adding the user to the group kvm (http://www.funtoo.org/KVM), kvm is still not able to start. It turned out that the group kvm has no rights to access /dev/kvm:
crw------- 1 root root 10, 232 18. Okt 22:28 /dev/kvm
After changing this to
crw-rw---- 1 root kvm 10, 232 18. Okt 22:28 /dev/kvm
Maybe just adding that change to the qemu package?