qapi: allow a 'gen' key to suppress code generation

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/scripts/qapi-commands.py b/scripts/qapi-commands.py
index f7def16..54d1f5d 100644
--- a/scripts/qapi-commands.py
+++ b/scripts/qapi-commands.py
@@ -405,6 +405,7 @@
 
 exprs = parse_schema(sys.stdin)
 commands = filter(lambda expr: expr.has_key('command'), exprs)
+commands = filter(lambda expr: not expr.has_key('gen'), commands)
 
 if dispatch_type == "sync":
     fdecl = open(h_file, 'w')