Here's how to change the boot order of a guest OS in VirtualBox. It is indeed possible to set up your guest OS to boot from CD/DVD-ROM, HDD or network (as long as these features are supported).

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 ...