Basically, we cannot create an image descriptor without any types
defined, and we cannot create any types without a default image
descriptor. I broke the circle by adding a stub pc type into the
database fill file so that all Emulabs start with a predefined type.
Then you can load the descriptor file, create your actual types, and
then go back and edit the image descriptors to set the types those
images run on (you always had to do this).