Não é possível iniciar o convidado do KVM

Question

Eu uso o Virtual Machine Manager (virt-manager).
Host - Ubuntu 18.04.
Convidado - Windows. Após o primeiro desligamento, não consigo iniciá-lo.

Mensagem de erro:

Error starting domain: Requested operation is not valid: network 'default' is not active

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 89, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 125, in tmpcb
    callback(*args, **kwargs)
  File "/usr/share/virt-manager/virtManager/libvirtobject.py", line 82, in newfn