Add 'thread_id' to the virDomainIOThreadIDDef as a means to store the
'thread_id' as returned from the live qemu monitor data.

Remove the iothreadpids list from _qemuDomainObjPrivate and replace with
the new iothreadids 'thread_id' element.

Rather than use the default numbering scheme of 1..number of iothreads
defined for the domain, use the iothreadid's list for the iothread_id

Since iothreadids list keeps track of the iothread_id's, these are
now used in place of the many places where a for loop would "know"
that the ID was "+ 1" from the array element.

The new tests ensure usage of the <iothreadid> values for an exact number
of iothreads and the usage of a smaller number of <iothreadid> values than
iothreads that exist (and usage of the default numbering scheme).