v1.0.3 bugfix: use ide bus for cdrom device.

Previously I use usb bus. In that case, sometimes cloud-init can't find the
datasource at boot time because when / is mounted, usb device may not be
ready. When that happened, cloud-init will fall back to None ds, which make
qvm's metadata fail.

When ide bus is used, cdrom is available much earlier. You can check it using
dmesg.