Very nice!
Is there a standard spot to put the build descriptor file in the source tree, or a standard name for it? I'd like to commit the build descriptor file.
Since you guys would be the first real users of the VM build process, you can influence the standard.

Maybe it can look in multiple locations, such as SRC/bitcoin.gitian-desc, SRC/gitian/bitcoin-desc.yml. Do you have a preference?
The build descriptor will need to change slightly. The commit hash has to be removed if you are committing it into git. Also, need a bit more added to it to support creation of the whole .tar.gz file. The latter requires that the build process be run twice, once with a 32 bit VM and once with a 64 bit one. I tried compiling for 32 on a 64 bit VM, but there's no 32 bit dev packaging for gtk et al.
In any case, I should be able to tie up the loose ends in the next couple of days.