Currently, on device detach, we parse given XML, find the device
in domain object, free it and try to restore security labels.
However, in some cases (e.g. usb hostdev) parsed XML contains
less information than freed device. In usb case it is bus & device
IDs. These are needed during label restoring as a symlink into
/dev/bus is generated from them. Therefore don't drop device
configuration until security labels are restored.