domain_conf: Add support for iothreads in disk definition
    
    Add a new disk "driver" attribute "iothread" to be parsed as the thread
    number for the disk to use. In order to more easily facilitate the usage
    and configuration of the iothread, a "zero" for the attribute indicates
    iothreads are not supported for the device and a positive value indicates
    the specific thread to try and use.