virt: introduce a XML processing module

this patch bootstrap a XML processing module for the
virt package, by extracting the core class:

virt.utils.XMLElement -> virt.vmxml.Element

The module is named 'vmxml' and not just 'xml'
to avoid name clashing with python stdlib inside
the new module, (see imports), and because just
'xml' seem a bit too generic here.

Change-Id: I77dd048067b8abcb77f4b9bf55fbfd0535672996
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26855
Reviewed-by: Dan Kenigsberg <danken@redhat.com>