Use template's OS id if no OS id is specified.

Still falls back to Other (64-bit) if needed.

This is to deal with the fact that I need to build a bunch of images for a bunch of OSes, and it's a pain to manually set that in each JSON file, especially since Other (64-bit) won't work for a bunch of OSes (i.e., they just won't boot - PV-required ones, basically).