Update "listextensions" controller function so that it expects arch/os/revision

Doing so will avoid returning a mix of extensions belonging to different
"arch", "os" or "revision". When using the interface within midas directly,
the user won't be confused.
Merge pull request #35 from jcfr/expect-nonempty-os-arch-revision

Update "listextensions" controller function so that it expects arch/os/r...