The virtual machine may fail to boot from an ISO because the boot order prioritizes the virtual hard disk over the optical drive, resulting in a “No bootable medium ...
I don't think Virtualbox has a UEFI option, so I think virtualbox is (rightly) booting legacy. But still it can't boot from that ISO I just tried VMWare Player I notice that one can choose UEFI or not ...
VirtualBox is software that runs on Windows, OS X, Linux, and Solaris, capable of full virtualization. Full virtualization allows VirtualBox to run OSes entirely different than the OS it is running on ...