VirtualBox builder: make VRDE configurable

This patch introduces three new attributes for the VirtualBox builder
to control the Virtual Remote Desktop Extension. Details are available
at http://www.virtualbox.org/manual/ch07.html.

vrde         bool    Enables/Disables the Virtual Remote Desktop Extension
vrdeaddress  string  Address of the host network interface the VRDE
                     server will bind to
vrdeport     uint    A port / port range the VRDE server can bind