Default network to libvirt NAT with DHCP and DNS

[cbosdo]

Since Avahi is working only part of the time, it would be nice to implement https://github.com/uyuni-project/uyuni/wiki/Libvirt-DNS-and-DHCP-without-Avahi into a default libvirt network.
This would help newcomers to get rid of the DNS issues.

The generated NAT network should be prefixed with the machine's prefix to avoid hijacking other networks.

[Bischoff]

Probable to do list:

• auto-generate MACs for our various VMs (client, server, etc.)
• create the new libvirt network in NAT mode, ensuring the previous setups like bridged setup still work
• populate the DHCP tables of dnsmasq with the MACs and corresponding IPs
• poulate the DNS tables of dnsmasq with the IPs and corresponding domain names

Helpful doc: https://registry.terraform.io/providers/dmacvicar/libvirt/latest/docs/resources/network