add xmlAttributeMap to serialized values

src/JMS/Serializer/Metadata/VirtualPropertyMetadata.php

@@ -59,6 +59,7 @@ class VirtualPropertyMetadata extends PropertyMetadata
             $this->xmlValue,
             $this->xmlKeyValuePairs,
             $this->xmlElementCData,
+            $this->xmlAttributeMap,
             $this->getter,
             $this->setter,
             $this->inline,
@@ -84,6 +85,7 @@ class VirtualPropertyMetadata extends PropertyMetadata
             $this->xmlValue,
             $this->xmlKeyValuePairs,
             $this->xmlElementCData,
+            $this->xmlAttributeMap,
             $this->getter,
             $this->setter,
             $this->inline,