Comment # 1 on bug 1001541 from Bernhard Wiedemann

in your brctl output, virbr0 is not connected to your LAN
and it is usually created by libvirt e.g. via
virsh net-start default

and why would one want to not just block, but penalize STP?
It helps to avoid network-loops on layer2

you could try
virsh net-edit default

and make sure you have a line there like
<bridge name='virbr0' stp='off' delay='0'/>