[ONOS-5884]YANG Serializer: Implement XML serializer.

Change-Id: Idffda88938d9c6c7b7c7814127a340bd2dc35039
diff --git a/runtime/src/test/resources/xmlAttributes.yang b/runtime/src/test/resources/xmlAttributes.yang
new file mode 100644
index 0000000..f9f3f93
--- /dev/null
+++ b/runtime/src/test/resources/xmlAttributes.yang
@@ -0,0 +1,23 @@
+module attributes {
+    yang-version 1;
+    namespace "http://example.com/schema/1.2/config";
+    prefix "attr";
+    container top {
+        container interface {
+            leaf name {
+                type string;
+            }
+            leaf mtu {
+                type string;
+            }
+            container address {
+                leaf name {
+                    type string;
+                }
+                leaf prefix-length {
+                    type string;
+                }
+            }
+        }
+    }
+}
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