* replace heartbeat with health-check
* instead of steady stream of newlines every 5s, now awaits input
and responds with `vmstat` output
* more reliably use host/config/vm-exec.json to get libvirt domain name
for snapshot-revert, server address, serial ports, vagrant setting, etc
* use iptables/nftables to disable networking
* added guest/bin/disable-network-[iptables,nftables]
* added guest/bin/enable-network-[iptables,nftables]
* replace ugly ___OUTPUT___ texts in sh, bash, ksh, zsh languages
* documentation updates and tweaks
If the output from a code snippet is `0` without a newline,
Perl treats this as a false value. When Guest::process_command()
returns a false value, that signals to guest-server::serial_server()
that it is the parent returning and so it will not send the result back
to the host.
However, `0\n` is a true value. Solution: always append a final newline to
the guest output.
