Vagrant & VirtualBox enable serial port sample

Vagrantfile

# -*- mode: ruby -*-
# vi: set ft=ruby :

Vagrant.configure("2") do |config|
  # ...
  
  config.vm.provider "virtualbox" do |vb|
    # ...
    
    vb.customize ["modifyvm", :id, "--uart2", "0x2F8", 3]
    vb.customize ["modifyvm", :id, "--uartmode2", "/dev/ttyS0"]
    
    # ...
  end
  
  # ...
end

# References:
# - https://www.linuxquestions.org/questions/slackware-14/virtualbox-serial-port-setup-frustration-586971/
# - https://gist.github.com/gijs/d0976c2630b996659330