Add the functorised functions to the RRDD interface.

Signed-off-by: John Else <john.else@citrix.com>
Hook the old API calls up to the functorised versions.

Signed-off-by: John Else <john.else@citrix.com>
Remove unneeded code.

Signed-off-by: John Else <john.else@citrix.com>
Implement registration of rrdd plugins by xenopsd.

Signed-off-by: John Else <john.else@citrix.com>
Refactor.

Signed-off-by: John Else <john.else@citrix.com>