Improve backwards compatibility of VGPU.create

If a client doesn't specify a VPGU_type when creating a VGPU, the server
will send us the default value of Ref.null. Here, we convert this into
the passthrough VGPU_type.

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #1402 from johnelse/vgpu-create

Improve backwards compatibility of VGPU.create