Support multiple xenopsds, running at the same time

* configure the list of xenopsds via xapi.conf
* configure the default xenopsd via xapi.conf
* allow per-VM xenopsd overrides via VM.other_config:xenops
* stop "synchronising" domain 0's state with xenopsd (if domain 0's state ever changes, the system falls apart!)
* remove ocaml/xenops: this duplicates xenopsd's repo
* remove ocaml/rrdd: this duplicates xcp-rrdd's repo

Note: this PR is a placeholder. More testing is required.