Add a type for registration of inter-domain RRD plugins.

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>
Add debugging

Signed-off-by: John Else <john.else@citrix.com>
This doesn't build.

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