$ qemu-system-ppc \
  -m 1024 \
  -boot d \
  -hda powerpc32.img \
  -initrd initrd.gz -kernel vmlinux \
  -append "cdrom-detect/try-usb=true" \
  -usb -usbdevice disk:debian-8.11.0-powerpc-DVD-1.iso \
  -no-reboot
WARNING: Image format was not specified for 'debian-8.11.0-powerpc-DVD-1.iso' and probing guessed raw.
 Automatically detecting the format is dangerous for raw images, write operations on block 0 will be restricted.
 Specify the 'raw' format explicitly to remove the restrictions.