qapi/types qapi/visit: Generate built-in stuff into separate files

Linking code from multiple separate QAPI schemata into the same
program is possible, but involves some weirdness around built-in
types:

* We generate code for built-in types into .c only with option
  --builtins.  The user is responsible for generating code for exactly
  one QAPI schema per program with --builtins.

* We generate code for built-in types into .h regardless of
  --builtins, but guarded by #ifndef QAPI_VISIT_BUILTIN.  Because all
  copies of this code are exactly the same, including any combination
  of these headers works.

Replace this contraption by something more conventional: generate code
for built-in types into their very own files: qapi-builtin-types.c,
qapi-builtin-visit.c, qapi-builtin-types.h, qapi-builtin-visit.h, but
only with --builtins.  Obey --output-dir, but ignore --prefix for
them.

Make qapi-types.h include qapi-builtin-types.h.  With multiple
schemata you now have multiple qapi-types.[ch], but only one
qapi-builtin-types.[ch].  Same for qapi-visit.[ch] and
qapi-builtin-visit.[ch].

Bonus: if all you need is built-in stuff, you can include a much
smaller header.  To be exploited shortly.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20180211093607.27351-21-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
[eblake: fix octal constant for python 3]
Signed-off-by: Eric Blake <eblake@redhat.com>