On testing the Newton commit, I'm seeing the issue described here: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1665698
Based on the wording of https://en.wikibooks.org/wiki/QEMU/Networking, I think by removing the <script path=''/>, qemu is reverting to a default of trying to run '/etc/qemu-ifup', which is not allowed by the Ubuntu apparmor shipped with libvirt.
On testing the Newton commit, I'm seeing the issue described here: https:/ /bugs.launchpad .net/ubuntu/ +source/ libvirt/ +bug/1665698
Based on the wording of https:/ /en.wikibooks. org/wiki/ QEMU/Networking, I think by removing the <script path=''/>, qemu is reverting to a default of trying to run '/etc/qemu-ifup', which is not allowed by the Ubuntu apparmor shipped with libvirt.