Code review comment for lp:~justin-fathomdb/nova/raw-disk-images

Thanks for the review Jay.

The 'mapped device not found' exception is actually tricky; it is expected if we can't read the partitions after loopback, which happens if we have a non-raw-disk-image (i.e. a 'sparse' format). The test is to try to make that case obvious. I'm not sure what we should do here, but currently the caller (at least for virtualbox) ignores errors when trying to inject the machine image. I've therefore left it as a raw exception.Error, but clarified the message to make the intent clearer.

I've added the TODO. Again, there's a big picture question of what we should be injecting and how we should be injecting it; hopefully this will become clearer as we start launching more OSes and the guest agent shapes up (which is required for Windows, I believe)

As for Cheetah, I have no particular reason to choose Cheetah vs any other templating system - it looked like decent syntax, the project seemed not-dead, and there's an Ubuntu package. I would suggest that we do need _a_ templating system, for complex templating or optional inclusions (here I use it so that the kernel and ramdisk XML elements are only output if they are needed). What that templating system is, I don't really care. The one datapoint I have is that Cheetah "works for me", but if anyone else has more experience...