Implemented XenAPI backend as a 'virt' __virtual__ module

First commit of xapi, a module to handle the Xen hypervisor through the XenAPI.

Big fat warning: the XenAPI used in this file is the one bundled with           
Xen Source, NOT XenServer nor Xen Cloud Platform. As a matter of fact it        
*will* fail under those platforms. From what I've read, little work is needed   
to adapt this code to XS/XCP, mostly playing with XenAPI version, but as        
XCP is not taking precedence on Xen Source on many platforms, please keep       
compatibility in mind.

This module has pretty much all the virt operations, with some Xen specific return values of arguments.