I experienced this problem while installing Funtoo Linux in a Virtualbox VM with EFI enabled. I used the standard Funtoo Installation Guide.

Problem

Virtualbox has volatile NVRAM: After a complete shutdown of the VM, it forgets the settings needed to find OS loader. See bug report https://www.virtualbox.org/ticket/14279.

Workaround

I created a startup.nsh script using the EFI shell to provide the path to the OS loader file.

Steps

1. remove any CD images (like systemrescuecd) from the VM settings
2. start the VM
3. wait for the countdown or press Esc immediately to reach the EFI shell
4. type edit startup.nsh to open the full-screen editor
5. enter "fs0:\EFI\Funtoo Linux [GRUB]\grubx64.efi" (including the double quotes, since the path contains spaces)
6. press CTRL-s (maybe twice) to save and confirm saving by pressing Enter
7. quit editor by pressing CTRL-q
8. type reset to reboot the machine (or kill the VM and boot it using the GUI)
9. wait for the countdown or press Enter to immediately run the startup.nsh script

Funtoo should now happily boot within the VM.