After importing a disk image you can access the connection details and look at the virtual networks. The default network for my VMs is:

<network connections="6">
  <name>default</name>
  <uuid>THIS_IS_GENERATED_RANDOMLY</uuid>
  <forward mode="nat">
    <nat>
      <port start="1024" end="65535"/>
    </nat>
  </forward>
  <bridge name="virbr0" stp="on" delay="0"/>
  <mac address="SOME_RANDOM_MAC"/>
  <ip address="10.11.12.1" netmask="255.255.255.0">
    <dhcp>
      <range start="10.11.12.128" end="10.11.12.254"/>
    </dhcp>
  </ip>
</network>

Once the default network is setup you need to start it and set it to auto-start:

virsh net-start default
virsh net-autostart default

Additionally there are some ufw rules that have to be added/updated to allow this to work. These go in files in /etc/ufw, here’s the group output for the rules from those files:

user6.rules:### tuple ### route:allow any any ::/0 any ::/0 in_virbr0
user6.rules:-A ufw6-user-forward -i virbr0 -j ACCEPT
user6.rules:### tuple ### route:allow any any ::/0 any ::/0 out_virbr0
user6.rules:-A ufw6-user-forward -o virbr0 -j ACCEPT
user6.rules:### tuple ### allow any any ::/0 any ::/0 in_virbr0
user6.rules:-A ufw6-user-input -i virbr0 -j ACCEPT
user6.rules:### tuple ### allow any any ::/0 any ::/0 out_virbr0
user6.rules:-A ufw6-user-output -o virbr0 -j ACCEPT
user.rules:### tuple ### route:allow any any 0.0.0.0/0 any 0.0.0.0/0 in_virbr0
user.rules:-A ufw-user-forward -i virbr0 -j ACCEPT
user.rules:### tuple ### route:allow any any 0.0.0.0/0 any 0.0.0.0/0 out_virbr0
user.rules:-A ufw-user-forward -o virbr0 -j ACCEPT
user.rules:### tuple ### allow any any 0.0.0.0/0 any 0.0.0.0/0 in_virbr0
user.rules:-A ufw-user-input -i virbr0 -j ACCEPT
user.rules:### tuple ### allow any any 0.0.0.0/0 any 0.0.0.0/0 out_virbr0
user.rules:-A ufw-user-output -o virbr0 -j ACCEPT