Comment 21 for bug 1432599

Well, the way to fix it properly is for NM to stop messing with devices that it has no business messing with :-). In the meantime, if adding the "unmanaged-devices" line to NetworkManager.conf is the only workaround to get you going, then definitely use it! (to be more precise - my intent wasn't to say that you should never do that, but rather that you shouldn't *have to* do that. Conversely, adding an ifcfg file for the libvirt network's bridge actually won't help matters at all (will actually make things even more broken), and so should never be done).

Have you tried removing that line with the most recent F21 updates? I have updates and updates-testing enabled on my F21 machine; while "nmcli list" does still show virbr0 and virbr0-nic as "connected" (implying that it is still attempting to "manage" them at some level), it at least hasn't messed with the IP address of virbr0, or disconnected virbr0-nic from virbr0 or tried to set it ~IFF_UP.