Refactor XMLElement to virt.utils

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

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1120246
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>
Reviewed-on: http://gerrit.ovirt.org/30071
Reviewed-by: Francesco Romani <fromani@redhat.com>