[vm] Adding Virtual Box version in azk doctor

[gullitmiranda]

eg:

$ azk doctor

      Version : 0.16.3
      Agent   : up
      Docker  : 1.8.1
      Use vm  : yes, ip: 192.168.51.4
      VirtualBox Version: 5.0.2r102096

the version should only be displayed if the azk are using VM:
https://github.com/azukiapp/azk/blob/master/src/cmds/doctor.js#L35-L38