Comment 6 for bug 247677

That's because virsh uses the root libvirtd and virt manager uses the userspace one.
The same result is achieved by running virt-manager with sudo.

Still, thanks a lot for the information because I was experiencing the same problem, but then on a personal Ubuntu server I access remotely. X-forwarding and sudo aren't the best of friends and so virsh is a very welcome alternative to virt-manager.

I do believe that this means that the userspace virt-manager is isn't able to bridge.
So I guessing that bug
https://bugs.launchpad.net/ubuntu/+source/virt-manager/+bug/192605 is still or again an issue.