The "Bootable" legacy ISO you've used is in fact not bootable, it relies on CDROM emulation provided by BIOS. VMs don't usually do this sort of emulation. If you set the guest OS to something like ...

For reasons more archival than practical, I've decided it's time to make a one-time backup of all my old floppies. While I may not ever need to use them again, I'd like to make .iso copies of my ...