Fix reset of cgroup when detaching USB device from LXC guests

Fix reset of cgroup when detaching USB device from LXC guests

When detaching a USB device from an LXC guest we must remove
the device from the cgroup ACL. Unfortunately we were telling
the cgroup code to use the guest /dev path, not the host /dev
path, and the guest device node had already been unlinked.
This was, however, fortunate since the code passed &priv->cgroup
instead of priv->cgroup, so would have crash if the device node
were accessible.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>