tools/ocaml/xenstored: drop select based socket watching

tools/ocaml/xenstored: drop select based socket watching

Poll has been the default since 2014, I think we can safely say by now
that poll() works and we don't need to fall back to select().

This will allow fixing up the way we call poll to be more efficient
(and pave the way for introducing epoll support):
currently poll wraps the select API, which is inefficient.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>