KVM: impossible to start default virtual network

This issue stems from the fact that you defined a network that's using an interface that is already in use by another network (by copying the existing default network and essentially cloning it).

The default virtual network is "missing" because you're listing the session networks instead of the system networks.

You can verify which URI you're using by running

$ virsh uri qemu:///session 

If you specify that you want to see the system networks then you'll be able to see the default network:

$ virsh --connect qemu:///system net-list Name State Autostart Persistent -------------------------------------------- default active yes yes 

You can also use sudo which automatically connects to qemu:///system:

$ sudo virsh net-list --all Name State Autostart Persistent -------------------------------------------- default active yes yes 

If you want to use the system networks by default as a non-root user you can check out How do I list virsh networks without sudo?

If you want more information you can check out Red Hat's documentation on Connecting to the Hypervisor with virsh Connect. Even though this documentation is aimed at RHEL, it still applies to Debian.