- change "vid/did" to "vid:did", old form still supported for now;
- allow "vid:did/subvid:subdid" specification in system.conf, in
which case a device will be visible to a driver if the subsystem
VID/DID also match.
Change-Id: I7aef54da1b0bc81e24b5d98f1a28416f38f8b266