[ubuntu] Upgrade to artful has broken VirtualBox raw disk booting of Windows 10

I have posted this in the VirtualBox forum as Raw disk booting of Windows 10 broken with Ubuntu artful. As it's not clear whether this is an Ubuntu or VirtualBox issue, for convenience I replicate the issue here.

Summary: I had Windows 10 (guest) booting as a raw disk from Ubuntu 17.04 (host). With the upgrade to Ubuntu 17.10 (artful), VirtualBox goes to grub2, instead of booting Windows 10.

More detail: I have the following partitions:



The machine is a Dell XPS 15. Windows lives in /dev/nvme0n1p3, and Ubuntu in /dev/nvme0n1p7. From linux, I create a raw disk

This creates files ~/VirtualBox_VMs/win10pro.vmdk and VirtualBox_VMs/win10pro-pt.vmdk.

I change the owner of the files to my regular user, so that VirtualBox can access them

To create the VM:

1. I launch VirtualBox 5.1.30_Ubuntu r118389
2. Create a new VM named "Windows_10_Pro"
3. Increase RAM to 8192 MB
4. "Use an existing virtual hard disk file" -> ~/VirtualBox_VMs/win10pro.vmdk

In the new VM Settings, I make the following changes

1. System -> Enable EFI (special OSes only)
2. Display -> Enable 3D Acceleration
3. Display -> Enable 2D Acceleration
4. Display -> Video Memory = 256 MB
5. Storage -> win10pro.vmdk -> Solid-state Drive

When I click "Start" for the VM, however, it goes to a "GNU GRUB" screen:



This was working before upgrading to Ubuntu artful. "Start" would just launch Windows 10. When I saw that it had stopped working, I deleted the VM, created a new one with the steps above, but I still get the GNU GRUB screen.

Although Windows 10 doesn't boot from VirtualBox anymore, I can launch it from the initial dual boot menu when I switch the laptop on. So Windows 10 hasn't broken, and works as before.

Any ideas?