I'm running some tests with Ubuntu 22.04 (more specifically, the Xubuntu variant). I can install it with either EFI or BIOS (aka EFI disabled). The OS seems to install and boot fine either way. The installer always creates a GPT partition table and therefore always insists on creating a "Reserved BIOS Boot Area" (aka "bios_grub" partition) and an "EFI System partition", whether the VM is using EFI or BIOS. Even when the VM is using BIOS, the installer seems to install Grub in an EFI-compatible way, because I can actually switch the VM to EFI and it still boots (which I did not expect to work).
The only difference I found so far is the procedure for trying to boot into an optical disk image after the OS has been installed.
- Without EFI, I just add the disk image to the virtual optical drive. Since that drive appears before the HDD in the boot order (System / Motherboard setting), it boots the optical disk when media is present.
- With EFI, the boot order seems to be ignored. It always boots into the Linux installation on the HDD unless I press ESC very early in the boot sequence (almost instantly after the VM starts), which brings me to a text-menu (EFI shell?) shell where I can select the optical drive in order to boot from it.