conf: Optimize the iothreadid initialization

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.