安装Ubuntu16.04开启的设置为：UEFI
重启后，引导不起来了。
解决很简单：
Shell> FS0:
FS0:\> cd EFI
FS0:\EFI> mkdir boot
FS0:\EFI> cp ubuntu\grubx64.efi boot\bootx64.efi
注意，上图最后没有把boot的名字修改，需要修改为bootx64.efi才能默认起来。
VirtualBox tends to forget its EFI boot entries. I know of two solutions to this problem:
1、Move/rename the boot loader you’re using (probably EFI/ubuntu/grubx64.efi on the EFI System Partition (ESP)) to EFI/BOOT/bootx64.efi. This is the default/fallback filename, and so VirtualBox will boot from it by default if there are no other entries.
2、Use the VirtualBox EFI’s menus to locate the boot loader you’re using (again, probably EFI/ubuntu/grubx64.efi) and add it as a boot option. I don’t recall the precise steps, and the menus aren’t exactly user-friendly, but if you poke around in the menus, the options are there.