Create an RRDD plugin functor, and use it to create the local plugin module.

Signed-off-by: John Else <john.else@citrix.com>
Add skeleton Interdomain module.
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>
Write time until next RRD reading back to xenstore.

The RRD plugin will pick up this value and schedule its updates
accordingly.

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