Skip validate only on constant readonly fields

A field can be readonly by the domain inversion in the validate method. So it
should be validated again in case its domain inversion change and reset it a
non-readonly.
So only field that are always readonly can be skipped for the validation.

issue6448
review38341002

src/model.js

@@ -1122,7 +1122,7 @@
                     if (fields && !~fields.indexOf(fname)) {
                         continue;
                     }
-                    if (field.get_state_attrs(this).readonly) {
+                    if (field.description.readonly) {
                         continue;
                     }
                     if (~exclude_fields.indexOf(fname)) {