Allow VirtualBox to manage permissions of shared files

Specifying fmode/dmode parameters to VirtualBox makes it more difficult to use
NFS as the implementation for folder-sharing, and makes it too easy to write
Puppet code that causes an endless file permission back-and-forth between
Puppet and VirtualBox.

Change-Id: I1d4a184e627ef072126ed19d505de4c53efbe141