conf: Parse user supplied aliases
    
    If driver that is calling the parse supports user supplied
    aliases, they can be parsed even for inactive XMLs. However, to
    avoid any clashes with aliases that libvirt generates, the user
    ones have to have "ua-" prefix.
    
    Note, that some drivers don't have notion of device aliases at
    all. Also, in order to support user supplied aliases some extra
    checks need to be done (e.g. during hotplug). Therefore we can't
    just enable this feature for all the drivers. Thus we need a flag
    that drivers set to tell parsing code that they can handle user
    supplied device aliases.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>