Comment 6 for bug 435527

Markus,

This bug is about using attach and detach commands via libvirt, and not USB (hostdev) devices. Hostdev support will be added in 10.04. In the meantime, please see /etc/apparmor.d/abstractions/libvirt-qemu:
  # WARNING: uncommenting these gives the guest direct access to host hardware.
  # This is required for USB pass through but is a security risk. You have been
  # warned.
  #/sys/bus/usb/devices/ r,
  #/sys/devices/*/*/usb[0-9]*/** r,
  #/dev/bus/usb/*/[0-9]* rw,

If you uncomment the apparmor rules above, then stop and start your VM, USB should work fine. If not, please file a different bug.