[Fedora-xen] virbr0 comes from where?

[Tom Horsley]

Actually I'm on centos, but I figure it probably does things
similar to fedora. On centos I see a virbr0 bridge that gets
created and used in clients where I pick virtual network.

I've been trying to understand Xen networking, and since this
virbr0 on centos is doing exactly what I tried unsuccessfully
to do on a debian xen 3.2 system, I thought I'd look and
see how it works.

Easy to say, hard to do :-).

I can't find any place that creates the virbr0 bridge. I don't
see an ifcfg file to create it at boot time, and I just replaced
the brctl command with a shell script to log all info about
brctl calls, then invoke the original brctl, and no brctl command
is ever issued to create virbr0!

Where do it come from?