I see lots of documentation on how to manually set up a network bridge and then manually add vm's tun devices to these bridges and then write a shell script that glues it all together. lots of work, very manual, and not impressive.
On the other hand if you want to use NAT to KVM+libvirt VMs to the network you just click the new network button in the virt-manager gui and relax.
Am I missing "the easy way" of causing a VM to share the physical network with the host?
You're not missing anything. It's something that they are working to resolve but at this time you have to manually create your bridge. There are plenty of howto's available for the various distributions on how to create a network bridge.
Here's a good place to start on why and how network bridges work with libvirt.