Add the functorised functions to the RRDD interface.

Signed-off-by: John Else <john.else@citrix.com>
Use records types instead of tuples for interdomain RRD plugin interface

Signed-off-by: John Else <john.else@citrix.com>
Merge pull request #21 from johnelse/xenostats

Add new RRD plugin registration interface
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