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.
The vm-exec command now recognizes the following options:
-vm-serial -vm-heart -vm-cid -vm-vport -vm
The -vm option accepts a name and configures the serial,
heart, cid and vport appropriately.
The vm-exec command can be configured by /host/config/vm-exec.json.
