How to troubleshoot when stuck at "GRUB loading."?

[Anders Ingemann]

Hello everybody

I am developing the debian bootstrapper "build-debian-cloud"
(https://github.com/andsens/build-debian-cloud) and am having some
trouble getting grub to boot my debian installation.
Since I bootstrap from a host system (with chroot etc.) I had to use
some workarounds to get grub installed onto a loopback device
(http://ebroder.net/2009/08/04/installing-grub-onto-a-disk-image/).

The setup consists of a vdi image partitioned with GPT into boot,
root, swap (in that order).
I am unable to get past the "GRUB loading." message when booting the
image in VirtualBox.

I have a hard time figuring out what is wrong with the setup,
especially because the scenario is a bit off the beaten path (mounting
vdi as an network block device, bootstrapping, using dmsetup to fake a
real hdd etc.). So the usual "just run BootRepair" or "reinstall grub"
won't really help to create a stable bootstrapping process.
I have run Ubuntus Boot Repair system to check if anything was wrong,
but I can't seem to find anything. The output is here:
http://paste.ubuntu.com/6115737/

The setup can be fully reproduced by cloning my repo and running
`./build-debian-cloud manifests/virtualbox.manifest.json` (only tested
on debian wheezy).
Simply create a new virtual machine in VBox, attach the resulting
image and boot it.

I would appreciate any help you can offer