Replace libvirt driver's use of libxml2 with ElementTree
Replace libvirt driver's use of libxml2 with ElementTree

libxml2 has been rather awkward as a dependency, and ElementTree
is in the standard library, so this replaces all use of libxml2 with
ElementTree instead.

Change-Id: Ib99983d5b04cc9f899584bfd4335fa9f7ece3c40

n0ano/ganttclient