virt: move _DomXML into the xml module

Move virt.vm._DomXML into virt.vmxml.Domain
Fix paths and imports (e.g. vmxml.Element -> Element),
no functional changes.

Change-Id: I0e2cbcac1b0cabc4847baf58f42bbc12495ecb17
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29699
Reviewed-by: Dan Kenigsberg <danken@redhat.com>