Replace vix disk lib with ffi

Replace VixDiskLib "C" binding with FFI and DRb

This Pull Request adds code to replace the C-language binding for VMware's VixDiskLib
with an FFI- and DRb-based binding.  The DRb Client side code will start a new process to
run the DRb Server code.  This code will use FFI to communicate with the VixDiskLib.
There is currently code in here that will determine if we should run this version (the default)
or use the old "C" binding instead.  This has been left in for comparison testing and will be
removed eventually.

@roliveri @chessbyte @Fryguy @jrafanie  Please Review.
I have moved the original VixDiskLib classes into new file vixdisklib_server.rb and kept the DRb-related server-side code including the VDDKFactory class in VixDiskLibServer.rb.