Modify VixDiskLib client/server code to use the new ffi-vixdisklib 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 "miq_wrapper" within ManageIQ which applies
a top-level wrapper which interacts with the DRb server code.

The ffi-vix_disk_lib gem is available in a repo under manageiq/ffi-vixdisklib and is ready to be
built along with this code.

@Fryguy @roliveri @jrafanie @chessbyte  Please review for sanity in using the gem.
Renaming to VdlWrapper and changing the filename miq_wrapper.rb to vdl_wrapper.rb.  I wanted to keep it shorter than VixDiskLibWrapper .... hope thats ok with all.
cc @roliveri