Hyper-V Gen2 VM are EFI-only so to enable VHDX format we need to add
EFI partition to the image. There are two options: either do it
only for VHDX or flip the switch for all images. This change is
for the latter option.

I tested VMDK, VHD, VHDX images generated with this change with
following VM software with both EFI and legacy boot modes:

qemu x86_64 legacy: works
qemu x86_64 EFI: works
VirtualBox legacy: works
VirtualBox EFI: works
VMWare Fusion legacy: works
VMWare Fusion EFI: loader boots fine but when control is passed to the kernel there a firmware error. Probably a compatibility issue in our EFI code or a bug in VMWare.
Hyper-V Gen 1 (legacy-only): works
Hyper-V Gen 2 (EFI-only): works