Unable to do advanced networking w/ KVM and Xenial

Our existing code to create network devices for KVM machines is expecting to be able to name the devices inside the host (eth0, eth1, etc).

However, even though we set the "GUEST" tag in libvirt, it doesn't actually change the name of the interface inside the container reliably.

This means that when we try to write a custom /etc/network/interfaces it fails to start because the devices we've named don't match the actual devices inside the VM. We need to do something fancier like matching the devices based on their MAC addresses. (I've seen them called 'ens3' w/ Juju 2.1 and 'enp0s2' w/ Juju 2.2.)