qapi: Turn generators into modules

qapi: Turn generators into modules

The next commit will introduce a common driver program for all
generators.  The generators need to be modules for that.  qapi2texi.py
already is.  Make the other generators follow suit.

The changes are actually trivial.  Obvious in the diffs once you view
them with whitespace changes ignored.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180211093607.27351-8-armbru@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
[eblake: minor tweak to keep 'blurb' one line]
Signed-off-by: Eric Blake <eblake@redhat.com>