conf: make virDomainControllerFindUnusedIndex() more generally usable
    
    Make virDomainControllerFindUnusedIndex() a global function so that it
    can be used outside domain_conf.c (as well as higher up in
    domain_conf.c itself)/ Also make its DomainDef arg a const* so that
    functions which only have a const* to the domain can use it.