This will instantiate any NICs which live on a given bus type. Each bus is allowed *one* substitution (for PCI it's virtio → virtio-net-pci, for Xen it's xen → xen-net-device; no point in overengineering it unless we actually want more). Signed-off-by: David Woodhouse <dwmw@amazon.co.uk> Reviewed-by: Paul Durrant <paul@xen.org> Reviewed-by: Thomas Huth <thuth@redhat.com>