error: add error_get_class()

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>

error.c

@@ -64,6 +64,11 @@ bool error_is_set(Error **errp)
     return (errp && *errp);
 }
 
+ErrorClass error_get_class(const Error *err)
+{
+    return err->err_class;
+}
+
 const char *error_get_pretty(Error *err)
 {
     return err->msg;