Add Vgpu video_card option to Xenops_interface

Signed-off-by: John Else <john.else@citrix.com>
Add some vgpu global keys to xenops_interface.ml

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #22 from johnelse/vgpu-trunk-merge

Additions required for vGPU