Working:

qemu-system-x86_64 -enable-kvm -m 4G -cpu host -vnc :50 -smp 4 \
-netdev bridge,id=nic1,br=br4 -device virtio-net,netdev=nic1,mac=52:54:00:12:29:16 \
-netdev bridge,id=nic2,br=br50 -device virtio-net,netdev=nic2,mac=52:54:00:12:40:22 \
-netdev bridge,id=nic3,br=br51 -device virtio-net,netdev=nic3,mac=52:54:00:12:41:33 \
-netdev bridge,id=nic4,br=br52 -device virtio-net,netdev=nic4,mac=52:54:00:12:41:44 \
-netdev bridge,id=nic5,br=br52 -device virtio-net,netdev=nic5,mac=52:54:00:12:41:55 \
-netdev bridge,id=nic6,br=br52 -device virtio-net,netdev=nic6,mac=52:54:00:12:41:66 \
-netdev bridge,id=nic7,br=br52 -device virtio-net,netdev=nic7,mac=52:54:00:12:41:77 \
-netdev bridge,id=nic8,br=br52 -device virtio-net,netdev=nic8,mac=52:54:00:12:41:88 \
-netdev bridge,id=nic9,br=br52 -device virtio-net,netdev=nic9,mac=52:54:00:12:41:99 \
-drive file=/vm2-sas1/sasvms/obsd-testrouter.qcow2,if=virtio

Not Working:
qemu-system-x86_64 -enable-kvm -m 4G -cpu host -vnc :50 -smp 4 \
-netdev bridge,id=nic1,br=br4 -device virtio-net,netdev=nic1,mac=52:54:00:12:29:16 \
-netdev bridge,id=nic2,br=br50 -device virtio-net,netdev=nic2,mac=52:54:00:12:40:22 \
-netdev bridge,id=nic3,br=br51 -device virtio-net,netdev=nic3,mac=52:54:00:12:41:33 \
-netdev bridge,id=nic4,br=br52 -device virtio-net,netdev=nic4,mac=52:54:00:12:41:44 \
-netdev bridge,id=nic5,br=br52 -device virtio-net,netdev=nic5,mac=52:54:00:12:41:55 \
-netdev bridge,id=nic6,br=br52 -device virtio-net,netdev=nic6,mac=52:54:00:12:41:66 \
-netdev bridge,id=nic7,br=br52 -device virtio-net,netdev=nic7,mac=52:54:00:12:41:77 \
-netdev bridge,id=nic8,br=br52 -device virtio-net,netdev=nic8,mac=52:54:00:12:41:88 \
-netdev bridge,id=nic9,br=br52 -device virtio-net,netdev=nic9,mac=52:54:00:12:41:99 \
-netdev bridge,id=nic10,br=br51 -device virtio-net,netdev=nic10,mac=52:54:00:12:42:10 \
-drive file=/vm2-sas1/sasvms/obsd-testrouter.qcow2,if=virtio