Refactor XMLElement to virt.utils

This is needed to avoid cyclic dependencies when other files
need the xml helper class.

Change-Id: I74528a597ace8148adbb0295a810ba7972b1270c
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29630
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>