Hrm...
I think I might have sent the bug report too quickly. I tried
downgrading to vagrant-libvirt 0.8.0 after sending the report and the
problem did not go away (0.8.0 used to work for me).
I first tried creating the networks manually in virt-manager, but that
didn't help.
Then I tried clearing out networks in virt-manager and removing the
.vagrant cache directory in my vagrant project, but I'm still getting
the issue, so vagrant is not able to recreate networks.
So something else has changed in my local setup that causes this issue.
I don't see a recent upgrade to vagrant itself in the dpkg log. However,
I can see that a couple of libvirt-daemon-* packages were upgraded on
june 3rd.
I just tried to downgrade libvirt* packages to 8.4.0-1 but that did not
fix the problem either. Now I'm not sure what more I could look into for
this issue :(