A JBossXmlMapKey. Specifies which property in the entry class
represents the entry's key.
There must be only one property in the entry class annotated with
JBossXmlMapKey.
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 key property. The standard JAXB/JBossXB annotations
should be used for binding the property to the XML schema construct.