I have just faced this problem, on a Fedora 20 fresh install. All Fedora 19 works out of the box with the same setup. It seems a regression to me.

Raised https://bugzilla.redhat.com/show_bug.cgi?id=1022977.
Do Fedora maintainers raise upstream issues, or am I expected
to do that?

AFAICS the "fix" for this bug is only a documentation change on sysctl.d, no actual fix was applied. The only related bug still open is https://bugzilla.gnome.org/show_bug.cgi?id=710858, but it seems abandoned.

The fact is that if you follow http://wiki.libvirt.org/page/Networking#Fedora.2FRHEL_Bridging but keep NetworkManager instead of network to manage your bridges, it works perfectly on F19, but not on Fedora 20.

Based on this, I think that we should have at least an open bug addresing this problem. What do you think?

Regards,

--
BrunoJCM