* docs/hvsupport.html, docs/libvir.html: Document HV support
	  for virNodeGetFreeMemory and virNodeGetCellsFreeMemory.
	* src/libvirt.c: Add DEBUG to virNodeGetFreeMemory and
	  virNodeGetCellsFreeMemory.
	* src/xen_internal.c: virXenError* functions now take an
	  additional virConnectPtr argument, and set it where possible.