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-commands.py b/scripts/qapi-commands.py
index 7d93d01..34f200a 100644
--- a/scripts/qapi-commands.py
+++ b/scripts/qapi-commands.py
@@ -29,9 +29,7 @@
 def generate_command_decl(name, args, ret_type):
     arglist=""
     for argname, argtype, optional, structured in parse_args(args):
-        argtype = c_type(argtype)
-        if argtype == "char *":
-            argtype = "const char *"
+        argtype = c_type(argtype, is_param=True)
         if optional:
             arglist += "bool has_%s, " % c_var(argname)
         arglist += "%s %s, " % (argtype, c_var(argname))