It turns out that this interface is actually an "internal network" which you can connect your VM's to if you want them talking to each other directly, as opposed to any network which the host is also on.
To setup my VM's I just added another interface and connected it to the "lxcbr0" bridge, by adding the following lines to the configuration:
lxc.network.type = veth
lxc.network.flags = up
lxc.network.link = lxcbr0
lxc.network.hwaddr = 4a:49:43:49:79:ef
and then configuring the interface in the "interfaces" file:
iface eth1 inet static
Then I did the same to another VM and they were able to talk to each other.