qapi: add const prefix to 'char *' insider c_type()

It's ugly to add const prefix for parameter type by an if statement
outside c_type(). This patch adds a parameter to do it.

Signed-off-by: Amos Kong <akong@redhat.com>
Suggested-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/scripts/qapi.py b/scripts/qapi.py
index 86e9608..dc690bb 100644
--- a/scripts/qapi.py
+++ b/scripts/qapi.py
@@ -470,8 +470,10 @@
 def is_enum(name):
     return find_enum(name) != None
 
-def c_type(name):
+def c_type(name, is_param=False):
     if name == 'str':
+        if is_param:
+            return 'const char *'
         return 'char *'
     elif name == 'int':
         return 'int64_t'