interface xml: add <driver> subelement in an extendible way

This patch makes no functional change. A <driver> subelement is added to
an <interface> element iff requested by the "vhost" custom property.

Change-Id: I1514fa11734e8d7e71a8c4aa51733d5087c38828
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25389
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>