Add virtualbox plugin.
Add virtualbox plugin.

Plugin source:
https://github.com/benclark/oh-my-zsh-plugins/tree/master/virtualbox

Signed-off-by: Mariusz Fik <e22610367d206dca7aa58af34ebf008b556228c5@fidano.pl>

Fisiu/oh-my-zsh,Fisiu/oh-my-zsh,Fisiu/oh-my-zsh