qemu: Allow use of iothreads for disk definitions
    
    For virtio-blk-pci disks with the disk iothread attribute that are
    running the correct emulator, add the "iothread=iothread#" to the
    -device command line in order to enable iothreads for the disk as
    long as the command is available, the disk iothread value provided is
    valid, and is supported for the disk device being added