qapi: New module target.json

qapi: New module target.json

We can't add appropriate target-specific conditionals to misc.json,
because that would make all of misc.json unusable in
target-independent code.  To keep misc.json target-independent, we
need to split off target-dependent target.json.

This commit doesn't actually split off anything, it merely creates the
empty module.  The next few patches will move stuff from misc.json
there.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Message-Id: <20190214152251[email protected]>