I think I have my answer: The kernel is busted (or something isn't loaded that I need, but don't know about :-). I copied my Fedora 20 desktop 3.15.8-200.fc20.x86_64 kernel and /lib/module files to the centos7 KVM host, rebuilt grub.cfg, and rebooted into the 3.15.8-200 kernel, and with no other changes the UDP packet forwarding is now working perfectly. I guess it is time to make yet another bugzilla account and submit a bug...