[ubuntu] Where is KVM's default bridge config?

Hi,

after installing KVM on Ubuntu 12.10, there is a default bridge virbr0 on the system. Where is the configuration file for that?

Reason I am asking: I am trying to get a handle on KVM and LXC and most of the guides mention to setup a bridge from scratch. Now I see the Ubuntu kvm package already added one for me, but it doesn't appear to have it's config anywhere in /etc.

Any hints would be appreciated.

Thanks!