This is a lot easier than expected.
The bridge is defined in network/default.xml. The file location is OS dependant, but you should be able to find it either below /etc or /usr.
Disable it there and restart the libvirt daemon.
The same file type is used to make Libvirt know about your networking setup. So if you have a more complex setup you can define your bridges (etc) via the normal OS config files and create XML files to make them visible in libvirt.