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.