virt-manager has a way to create networks, I have one called
"vm_network" which uses NAT, I think that might be a default thing, and
it seems to work.

When you create a virtual machine, you should be able to select to use a
network. If I look at an existing machine I have, I see a "NIC ..."
device, which uses the "vm_network".

If you've got all this, it could be that the networking isn't getting
started in the virtual machine...