Sort out passthrough examples

[Ivan-Velickovic]

It's fairly useful to be able to an example of passing through a particular device, but I am not sure whether we want to have a separate example for each device.

Right now we have a examples/ethernet and examples/framebuffer that show off passing through and ethernet device and some sort of graphics device. We also have #32 for audio.

I think it makes sense to have all of these in one example, and so we should clean them up and merge them into a single example.