If you open a console session, then reboot the guest, you can hit Escape to bring up the boot menu, and select CD-ROM.
The time window is < 1 second, so it's easy to miss.
If you open a console session, then reboot the guest, you can hit Escape to bring up the boot menu, and select CD-ROM.
The time window is < 1 second, so it's easy to miss.