Comment 78 for bug 787091

Same or at least similar problem here.

VM runs is a clean Server ubuntu-12.04.2-server-amd64 with only ssh installed.

Host System is a Raring System also 64bit setup with virt-install
Apparmor is deinstalled so it won't disturb/distract
qemu.conf has two changes:
a) user= "+0", so that permission issues won't disturb the testcase.
b) cgroup_controllers = [ "cpu", "devices", "memory", "blkio", "cpuset", "cpuacct" ]

cgroup-lite is installled and mounts cgroups !

if i now use qemu monitor to passthrough a device by hostport like that(take sensible bus and port to reproduce):

add_device usb-host,hostbus=8,hostport=2

i see permission denied errors:
husb: open /dev/bus/usb/008/008: Operation not permitted
husb: open /dev/bus/usb/008/009: Operation not permitted

once i unmount all cgroup, everything works fine. the devices apear in the vm once they are pluged in.

i see the same thing happenning on other machinges with ubuntu-12.04.2 guest on ubuntu-12.04.2 host(fairly fresh install).
hth