Adding a virtual network

I've been at this for over an hour, reading forum posts and such. Can't find what I'm looking for. 

Goal: Set up a new virtual network that is not attached to the rest of the LAN. This way I can do some testing/goofing with PFsense and some Linux hosts on that new virtual network, free of anything else going on in my regular network. 

My understanding is that I need to simply create a new virtual network. Only way I could figure to do that was `virsh net-define <path to XML>`. But even once I create this new interface, activate it, etc (lab1 is my new interface): 

root@unraid:~# virsh net-list --all 
 Name      State      Autostart   Persistent
----------------------------------------------
 default   inactive   yes         yes
 lab1      active     yes         yes

It does not show up as one of the options when I try to spin up a new VM and select a network interface. Though I can see clearly that `ifconfig` finds it: 

lab1: flags=4099<UP,BROADCAST,MULTICAST>  mtu 1500
        inet 10.0.0.1  netmask 255.0.0.0  broadcast 10.255.255.255
        ether 52:54:00:e5:34:a7  txqueuelen 1000  (Ethernet)
        RX packets 0  bytes 0 (0.0 B)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 0  bytes 0 (0.0 B)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

And I guess my expectation must be off, which is that this new interface should show up under the list of interfaces under the 'network bridge' dropdown when creating/editing a VM. How would I connect my network interface to this new `lab1` network as opposed to one of the standard `virbr0` or `br0` "bridge" interfaces? Just raw XML definition?