libvirt::network can now enable/disable networks

we rework such that it uses virsh to enable or disable or autostart a
specific network. Please note that we don't manage any files anymore.

We have adapted the creation of the XML to be more akin to what libvirt
would create.
simplify handling of default network

add testing for that. n.b.: This is failing madly on centos, and I don't
know why yet.
Another issue is that when we need to idempotently recreate default.xml,
we don't know how.
really make sure files are absent & default is default

we declare the default network now in params, so we are able to
redeclare it idempotently.

further, we guarantee that the network files are *really* gone.
fix spec tests
libvirt *service* must run before network config

libvirt::network uses `virsh` to add or remove network configurations.
As such it is not only dependant on the `virsh` binary, but also on the
libvirt service to provide its functionality.