A JBossXmlMapValue. Specifies which property in the entry class
represents the entry's value.
If none of the properties in the entry class is annotated with
JBossXmlMapValue then an instance of the entry class
is considered the value.
Note: this annotation doesn't bind Java element to XML schema construct
directly but just signifies which property should be used as the map
entry's value property. The standard JAXB/JBossXB annotations
should be used for binding the property to the XML schema construct.