Comment 4 for bug 1990853

The relevant section in postinst output is:

Default libvirt network on 192.168.122.1/24 already taken
Changing to free 192.168.123.1/24
Enabling libvirt default network

1. It realized it would have a conflict (since it runs in another VM with default net)
2. It switched to a new network pattern that is free.
3. it enables the default network

That enabling is done using libvirts autostart feature:

set_autostart()
{
    echo "Enabling libvirt default network"
    if [ ! -e /etc/libvirt/qemu/networks/autostart/default.xml ]; then
        ln -s /etc/libvirt/qemu/networks/default.xml \
              /etc/libvirt/qemu/networks/autostart/
    fi
}

So there is issue #1.
The call to "maybe_set_autostart" needs to happen before all the service (re)starts.
That allows libvirt to pick up that the config was changed.
This was modified recently in debian and on merging did place it after the service handling.

Note: Upgrades are not affected, nor are any places that start the default network. We just miss the comfort that it is started right after installation of libvirt.