Comment 19 for bug 1432599

libvirt's bridge devices are never listed (and shouldn't ever be listed!) in an ifcfg file. They are created by libvirtd when it is started, and NM shouldn't be touching them, attaching things to them, or even displaying them (and it shouldn't take an "unmanaged-devices" line in /etc/NetworkManager/NetworkManager.conf to make that happen).

If you notice that NM is doing something with your virbrX bridges, don't try to fix it by creating an ifcfg file for that device, that will only make matters worse, as NM will now have tacit permission to manage the device, and even to create a new device with that name the next time the system boots (which will result in libvirt being unable to start its virtual network that uses that bridge).