If you installed on a UEFI system you may want to run the following commands before booting into your new installation:
efi="/target/boot/efi/EFI"; \ if [[ -e "${efi}/boot" ]]; then \ echo "Backup:"; \ mv -v "${efi}/boot/" "${efi}/boot-bak-$(lsb_release -cs)"; \ fi; \ echo "Copy:"; \ cp -va "${efi}/ubuntu" "${efi}/boot"; \ arch="$(ls -1 {efi}/ubuntu/shim*.efi)"; \ arch="${arch#*shim}"; \ arch="${arch%.efi}"; \ cp -v "${efi}/boot/shim${arch}.efi" "${efi}/boot/boot${arch}.efi";
This will copy Ubuntu's boot files to the default bootloader location and backup any previously existing files. By doing this you save yourself from headaches caused by non-standards compliant behavior of some UEFI firmwares found on VirtualBox as well as some older HP, Toshiba, Sony and other devices.