I was having a similar issue with my Virtio port showing down. You can fix this by clicking on the Shell icon on the top right, then typing this in:
virsh net-start default
That brought my interfaces up. Now I just need to get access to Unraid from the virtio network. I am using a PCI ethernet adapter as my WAN interface and then I want to use the Virtio-net interface for my LAN interface. So I will be troubleshooting this tonight. I started working on this last night at 7pm and went to bed at 2am this morning. I am running a HP Proliant DL380p G8 , I tried splitting up the 4 port NIC using ACS but unfortunately for what ever reason, all 4 ports have the same serial, so even if I split them into separate IOMMU groups, restart, then add each port individually to PFSense, when PFSense starts up, it just shows up as one port.
EDIT: Just to be clear, my Virtio-net is using my servers 4 port network card, the one I mentioned above: HP 4-port 684208-B21 Ethernet 1Gb 331FLR Adapter
The issue I have is that PFSense is a VM, so when Unraid is rebooted, PFSense starts after Unraid (Obviously), but due to this, Unraid marks the NICS as "Shutdown-inactive" because nothing is currently using the ports, this causes a chain effect, where PFSense looses connectivity to that port now when the VM auto starts because Unraid has turned that port off. I have to manually start the port, then reboot PFSense to see the port then redo the interface assignments again.
I might be doing something wrong, I will tackle this again tonight when I get home.
EDIT: I will document my steps and post my config if I manage to come right tonight.