conf: Add new domain XML element 'iothreadids'

conf: Add new domain XML element 'iothreadids'

Adding a new XML element 'iothreadids' in order to allow defining
specific IOThread ID's rather than relying on the algorithm to assign
IOThread ID's starting at 1 and incrementing to iothreads count.

This will allow future patches to be able to add new IOThreads by
a specific iothread_id and of course delete any exisiting IOThread.

Each iothreadids element will have 'n' <iothread> children elements
which will have attribute "id".  The "id" will allow for definition
of any "valid" (eg > 0) iothread_id value.

On input, if any <iothreadids> <iothread>'s are provided, they will
be marked so that we only print out what we read in.

On input, if no <iothreadids> are provided, the PostParse code will
self generate a list of ID's starting at 1 and going to the number
of iothreads defined for the domain (just like the current algorithm
numbering scheme).  A future patch will rework the existing algorithm
to make use of the iothreadids list.

On output, only print out the <iothreadids> if they were read in.