I am using VirtualBox quite a bit (I occasionally go qemu from time to time), especially to test my live Linux builds. I now mostly setup my virtual machines to have the EFI boot enabled. However, whenever I do an installation, the machine will most of the time stop at a UEFI shell prompt.

To boot the previously installed system, you need to identity your EFI partition (e.g. fs0) from the listed devices and look for the path to the bootloader (e.g. grubx64.efi). You can type (these are case insensitive),

    FS0:
    \EFI\ubuntu\grubx64.efi

It should now boot. But, you will still get that prompt on the next boot. To make it permanent,

    FS0:
    edit startup.nsh

That will start a text editor. Write exactly what we typed before to boot. Press <Ctrl+s> and <Enter> to save, and <Ctrl+q> to quit. Now, your bootloader should execute when the system starts.