vm: use vmxml.Element in setLinkAndNetwork

Use vmxml.Element instead of xml.dom.minidom.Element
in vm.setLinkAndNetwork.
No functional changes.

Change-Id: I92f195c6a079ca35ab8f2ed0e6677b0f9a41511d
Signed-off-by: Francesco Romani <fromani@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/29696
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>