qapi script: clean up in scripts

This patch improve docs and uses c_type(argentry, is_param=True)
in script.

Signed-off-by: Wenchao Xia <wenchaoqemu@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/scripts/qapi.py b/scripts/qapi.py
index 54b97cb..f2c6d1f 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -255,7 +255,7 @@
             if structured:
                 raise QAPIExprError(expr_info,
                                     "Nested structure define in event is not "
-                                    "supported now, event '%s', argname '%s'"
+                                    "supported, event '%s', argname '%s'"
                                     % (expr['event'], argname))
 
 def check_union(expr, expr_info):