qemu: Use 'niothreadids' instead of 'iothreads'

qemu: Use 'niothreadids' instead of 'iothreads'

Although theoretically both should be the same value, the niothreadids
should be used in favor of iothreads when performing comparisons. This
leaves the iothreads as a purely numeric value to be saved in the config
file.  The one exception to the rule is virDomainIOThreadIDDefArrayInit
where the iothreadids are being generated from the iothreads count since
iothreadids were added after initial iothreads support.