Configuring by :interface doesn't work very well because Vagrant has
no idea about what interfaces are present in the VM, for example if
the image has 'docker' installed but not biosdevname, then
interface_names[0] = "docker0" which is usually not what you want
mapped to the first network from the Vagrantfile.
So if the plugins (like vagrant-libvirt) or the Vagrantfile has
given us a network with a MAC address, use that to find the interface
name for the network. Otherwise use slot numbers as before.
49c4581a8c