Bugfix: xml network definition.

In case of a bridgeless network, interface xml element should be
contained within the forward element, it should not be a direct
child of the network element like in the case of a bridged network.

Change-Id: I89b9723367aa92318de7d6587fef7f01e73a58f6
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17085
Reviewed-by: Giuseppe Vallarelli <gvallare@redhat.com>