Support XML marshaling of Configuration instances

Infinispan supports a very flexible way of configuring caches using multiple levels of inheritance.
While this is really nice, it is sometimes not easy to understand how exactly a particular cache was configured.
(E.g. JPA is just one example where a cache configuration can be adjusted in several ways).

JMX or debuggers help but may not always be available.

What about allowing to convert a configuration instance back to XML to give a quick overview of the really applied settings. (Optionally it might also be a nice little feature to implement a "toXmlString();" method or override the default and also make this available through JMX)

JAXB annotations are already present on all configurable options but a couple getters and a @XmlRootElement annotation is missing that would allow to convert an existing instance back to XML. The effort in doing this should be rather low.

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This feature request may also fit with #ISPN-791 as it would allow to transfer cache configs using XML.