Had some problems with a bridged network not allowing traffic (dhcp) to and from guests through the assigned interface(s).

sysctl -w net.bridge.bridge-nf-call-iptables=0
May require sysctl -p as well, not sure.

Add net.bridge.bridge-nf-call-iptables=0 to /etc/sysctl.conf
After that the vm guests got network access restored. Don't know why it stopped working.

Answered here…
https://serverfault.com/questions/1113764/libvirt-with-qemu-guest-bridged-networking-does-not-work