VirtualBox shared folder support for FreeBSD guests

by Oleksandr for FreeBSD

VirtualBox is very popular virtualization product which supports a large number of operating systems. And also has many other features, one of main is "share folders". It is used to transfer files using only internal resources of the system, without network connection. FreeBSD does't have support this features with guests OS,and it is uncomfortable when using VirtualBox, that's why this project is very useful for users and interesting to develop for me.