conf: Optimize the iothreadid initialization
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1264008
    
    The existing algorithm assumed that someone was making small, incremental
    changes; however, it is possible to change iothreads from 0 (or relatively
    small number) to some really large number and the algorithm would possibly
    spin its wheels doing unnecessary searches.
    
    So, optimize the algorithm using a bitmap to find available iothread_id's
    starting at 1 that aren't already defined by a "<thread id='#'>" and
    filling in the iothreadids array with those iothread_id values.