qapi: Streamline boilerplate comment generation

qapi: Streamline boilerplate comment generation

Every generator has separate boilerplate for .h and .c, and their
differences are boring.  All of them repeat the license note.

Reduce the repetition as follows.  Move common text like the license
note to common open_output(), next to the existing common text there.
For each generator, replace the two separate descriptions by a single
one.

While there, emit an "automatically generated" note into generated
documentation, too.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Message-Id: <20180211093607[email protected]>
Signed-off-by: Eric Blake <[email protected]>