qapi: don't convert enum strings to lowercase

Next commit will introduce enum strings in camel case.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>

scripts/qapi-types.py

@@ -70,7 +70,7 @@ const char *%(name)s_lookup[] = {
         ret += mcgen('''
     "%(value)s",
 ''',
-                     value=value.lower())
+                     value=value)
 
     ret += mcgen('''
     NULL,