Modify VixDiskLib Client/Server to Use new ffi-vix_disk_lib gem

Change the VixDiskLib client/server DRb code to use the ffi-vix_disk_lib gem instead
of the existing code in vixdisklib_api, vixdisklib_server, and the contents of the
VixDiskLib_FFI directory.

Two different interfaces to the new gem exist via the "api" layer - direct to the vmware library,
and the "api_wrapper" layer, which performs ffi memory management functionality.

This commit changes an existing file to a "vdl_wrapper" within ManageIQ which applies
a top-level wrapper which interacts with the DRb server code.

The ffi-vix_disk_lib gem has been added to rubygems.org (version 1.0.1) and can be used by this
code.

@Fryguy @roliveri please review.
Merge pull request #850 from jerryk55/ffi-vixdisklib-gem

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