This requires drivers to opt in to handle the raw modelstr
network model, all others will error if a passed in XML value
is not in the model enum.

Enable this feature for libxl/xen/xm and qemu drivers
Acked-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>