kvm virtio disk mystery solved

I suppose this is what I get for depending on a GUI - something I normally never do.

kvm stores its guest definitions in an xml file in /etc/libvirt/qemu.  So to see what adding a new drive would look like in that file, I went to virt-manager, fired up the server, clicked the 'hardware' tab, and installed a virtio disk.  Then, I went looked at the file, and...

No difference.  I even saved it and did a diff.  No difference. OK, it must need a reboot - makes sense, kinda.  Rebooted the machine; no vda disk, no change to the .xml.  All right, I shut the guest down completely, get out of virt-manager, get back in... nada.  The disk is showing up in the hardware tab, but not in the .xml.  Nothing in the logs.

Feeling mightily confused, I fired off a support request to Red Hat and started messing around.  I removed the disk from the guest, shut the guest down, added the disk - AHA!  That did it!  Started up the guest; the disk shows up.  All is well.

This sure doesn't sound like the behavior I'd expect.  I mean, if you're going to have a GUI, why not an error message if you try to do this?  I think I'll submit a bug report.