qapi: New classes QAPIGenC, QAPIGenH, QAPIGenDoc

These classes encapsulate accumulating and writing output.

Convert C code generation to QAPIGenC and QAPIGenH.  The conversion is
rather shallow: most of the output accumulation is not converted.
Left for later.

The indentation machinery uses a single global variable indent_level,
even though we generally interleave creation of a .c and its .h.  It
should become instance variable of QAPIGenC.  Also left for later.

Documentation generation isn't converted, and QAPIGenDoc isn't used.
This will change shortly.

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: <20180211093607.27351-6-armbru@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
[eblake: fix nits spotted by Michael]
Signed-off-by: Eric Blake <eblake@redhat.com>

scripts/qapi-introspect.py

@@ -179,21 +179,23 @@ for o, a in opts:
 
 blurb = ' * QAPI/QMP schema introspection'
 
-(fdef, fdecl) = open_output(output_dir, do_c, do_h, prefix,
-                            'qmp-introspect.c', 'qmp-introspect.h',
-                            blurb, __doc__)
+genc = QAPIGenC(blurb, __doc__)
+genh = QAPIGenH(blurb, __doc__)
 
-fdef.write(mcgen('''
+genc.add(mcgen('''
 #include "qemu/osdep.h"
 #include "%(prefix)sqmp-introspect.h"
 
 ''',
-                 prefix=prefix))
+               prefix=prefix))
 
 schema = QAPISchema(input_file)
 vis = QAPISchemaGenIntrospectVisitor(opt_unmask)
 schema.visit(vis)
-fdef.write(vis.defn)
-fdecl.write(vis.decl)
+genc.add(vis.defn)
+genh.add(vis.decl)
 
-close_output(fdef, fdecl)
+if do_c:
+    genc.write(output_dir, prefix + 'qmp-introspect.c')
+if do_h:
+    genh.write(output_dir, prefix + 'qmp-introspect.h')