I can confirm the issues. It seems it is related to:

https://fedoraproject.org/wiki/Changes/Vagrant_2.2_with_QEMU_Session#Upgrade.2Fcompatibility_impact

IOW using system instead of user session is workaround:

~~~
  config.vm.provider :libvirt do |libvirt|
    libvirt.qemu_use_session = false
  end
~~~

However, I wonder what is so specific about Fedora cloud images network settings, which would trigger this issue? Any idea?

In general to this upgrade, which is what probably happened, anything associated with your previous VM instances(and the instances themselves) can't be accesible with the upgraded Vagrant version  unless you change the current default setting, as Vit mentined.

That is because it newly uses unprivilleged libvirt access `qemu:///session`, instead of the default, privilleged, `qemu:///system`.

Therefore anything that was created before the upgrade is still in the `qemu:///system` domain and unaccessible via `qemu:///session`. Newly created, or re-created, images should be fine.

The following could possibly also help:
  - destroy the instance before upgrade and create new with new version
  - change fedora29 in `config.vm.define "fedora29"` to some other string, to be non-conflicting
  - use `virsh` to list/modify/remove the libvirt `objects` manually (`sudo virsh` and `virsh` also have different access)

Unfortunatelly I know of no way we can safely port all images to `qemu:///session` for this upgrade to be transparent to users. Therefore user needs to re-create the images or the switch off qemu_use_session value. The upgrade is not expected to be seamless, but I agree the traceback message is awful. Maybe that could be fixed?

Something like this happens to me on latest F31. I've noticed that it only happens when I have a second disk `domain.storage :file, :size => '20G'` in my Vagrantfile:

```
vagrant-2.2.5-1.fc31.noarch
vagrant-libvirt-0.0.45-3.fc31.noarch
```

```
         2: from /usr/share/vagrant/gems/gems/vagrant-libvirt-0.0.45/lib/vagrant-libvirt/action/create_domain.rb:147:in `call'                                                                                                        
         1: from /usr/share/vagrant/gems/gems/vagrant-libvirt-0.0.45/lib/vagrant-libvirt/action/create_domain.rb:147:in `each'                                                                                                        
/usr/share/vagrant/gems/gems/vagrant-libvirt-0.0.45/lib/vagrant-libvirt/action/create_domain.rb:156:in `block in call': undefined method `+' for nil:NilClass (NoMethodError)
```

```
Vagrant.configure(2) do |config|

    config.ssh.insert_key = 'true'
#   config.ssh.forward_agent = 'true'
#

    config.vm.synced_folder "/var/b/shared", "/var/b/shared", type: "sshfs"

    config.vm.provider :libvirt do |domain|
      domain.memory = 4096
      domain.cpus = 4
      domain.nested = true
      # Add an extra disk
      domain.storage :file, :size => '20G'
     #domain.volume_cache = 'none'
    end

    host = 'vanilla-f31'
    box  = 'fedora/31-cloud-base'

    config.vm.define host do | tmp |
        tmp.vm.hostname = host
        tmp.vm.box = box
    end
end
```

Just from browsing the title of the PR this may be fixed by upstream PR: https://github.com/vagrant-libvirt/vagrant-libvirt/pull/1030

I hit this issue too. Also, I think, along with the patch mentioned in comment 4, it would require fog-libvirt version 6 and above because of the following two changes
https://github.com/fog/fog-libvirt/releases

Allow to specify UID and GID of volume
Scope code under Fog::Libvirt instead of Fog::Compute::Libvirt

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