Modify VixDiskLib client/server code to use the new ffi-vixdisklib gem.

Change the VixDiskLib client/server DRb code to use the ffi-vixdisklib gem instead 
of the existing code in vixdisklib_api, vixdisklib_server, and the contents of the 
VixDiskLib_FFI directory.
    
The highest level ffi-vixdisklib wrapper, miq_wrapper, will be usedto mirror the previously 
existing functionality.  Remove previously used code which is replaced by the gem code.

This PR cannot be merged until the ffi-vixdisklib gem is available.  It is currently in the
jerryk55/ffi-vixdisklib  repo but should probably be added under manageiq/ffi-vixdisklib.

@Fryguy @roliveri @jrafanie @chessbyte  Please review for sanity in using the gem.