Comment 7 for bug 1784501

Nothing has changed regarding privileged vs. unprivileged settings.

I just set this up again with a privileged container, and I believe the cause of the regression to actually be in libvirt. I think it must have silently ignored the bridge configuration error before and marked the network active (such that it shows up in `virsh net-list` without the `--all` parameter).

Reasoning: yesterday before I rebuilt my test container, MAAS showed only two networks; none of which were virbr* interfaces (I never explicitly deleted the default virsh network). Today when I encountered this bug, MAAS showed three (because I made the container privileged). Additionally, MAAS KVM pods check to see if a `default` or `maas` network is active in virsh before allowing a KVM pod to be composed. Therefore, unless libvirt believed its `default` network was up and running, my previous test environment would not have worked at all.

Conclusion: we're just now seeing this because libvirt (in bionic-updates) began raising an error and failing to mark a network active in the case that it could not configure the bridge STP parameters.