Move update_portgroup from postive cases to negative cases

[qiankehan]

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