nova lxc cgroup /dev/kvm

well, i install openstack with lxc(nova-compute-lxc),when i launch a lxc vm,i can not mknod /dev/kvm in the lxc instance.
Then, i check the cgroup in /sys/fs/cgroup
root@superstack:/sys/fs/cgroup/devices/libvirt/lxc/instance-00000035# cat devices.list
c 1:3 rwm
c 1:5 rwm
c 1:7 rwm
c 1:8 rwm
c 1:9 rwm
c 5:0 rwm
c 5:2 rwm
c 136:* rwm

as we can see,there is no "c 10:232 rwm" record, and i can execute this command
echo "c 10:232 rwm" >> devices.allow
then i can "mknod -m 666 /dev/kvm c 10 232" in the lxc instance
but when i reboot the lxc, the record "c 10:232 rwm" disappeared.

i want to know where to conifg the default cgroup for lxc

#######i know how to config cgroup for kvm/qemu########
vi /etc/libvirt/qemu.conf
......
cgroup_device_acl = [
    "/dev/null", "/dev/full", "/dev/zero",
    "/dev/random", "/dev/urandom",
    "/dev/ptmx", "/dev/kvm", "/dev/kqemu",
    "/dev/rtc", "/dev/hpet","/dev/net/tun",
]
......

service libvirt-bin restart

##################################
but when i configed the /etc/libvirt/lxc.conf like qemu.conf, it did't do the good job like qemu.conf

i guess when i "nova boot lxc", it did't read this configuration,so i do following jobs
#########################
 mv /etc/libvirt/lxc.conf{,.bak}
service libvirt-bin restart
nova boot .....

and i can boot the lxc instance and see the same cgroup