Move update_portgroup from postive cases to negative cases

Since https://gitlab.com/libvirt/libvirt/-/commit/c3302ceb1d (libvirt 10.6.0), the update of portgroup is forbiden. Please change the case from  **virtual_network.iface_update.positive_test.update_portgroup** to **virtual_network.iface_update.negative_test.update_portgroup**