XML: Add darwin device file for real.
SCons: Adapt device string for hosted.

Instead of using `${hosted}`, which is a magic string that defies logic
and breaks minds, we now simply use `hosted` for a project that should
run on any hosted system.
This will use the linux, darwin or windows device file, depending on
the underlying OS.

You may also force a specific device file, by explicitly specifying it:
- `hosted/linux` for linux,
- `hosted/darwin` for darwin, and
- `hosted/windows` for windows.