Removed all traces of XML, OVA and OVF rel. code

* XML, legacy code, not maintained -- removed
* OVA, legacy code, not maintained -- removed
* OVF, legacy code, not maintained -- removed
* Removed dependency on Nokogiri
* XML parser and renderers will be re-implemented in the near
  future
Minor version bump to 4.2.10

This release removes unused/unmaintained code handling XML,
OVA and OVF. Since most of it was already broken or untested,
I decided to remove it ASAP, in a minor version update.