Libvirt driver to_xml(...) method now takes the context

Fixes OS-237

The libvirt driver method to_xml(...) now takes the context as
the first argument. Also removed the grizzly specific code that
handled the interface change between grizzly minor releases because
havana uses the newer version only.

Change-Id: I68998479dfe1c58bb5a90f37786aaae40b44b97e