providers/virtualbox: Add 'post-comm' to the list of valid events

Commit 6a5fee0 is considered as fix for [GH-3080]

Actually, the new customization event has been added - `post-comm`. But it doesn't work because this event is not passing the validation (`virtualbox/config.rb#validate`). 

So, we just need to add this event to the list of valid events :)