oxenstored: Build it

oxenstored: Build it

 * Add the relevant build dependencies

     ocaml-native-compilers is good on stretch because it
     will get us better output code.  In buster the
     ocaml-native-compilers package is merged into ocaml-nox.
     In bullseye we can drop ocaml-native-compilers from the list.

 * Drop the rules line that disables the ocaml build.

 * Ship /etc/xen/oxenstored.conf.

 * Placate dh_missing about ocaml development libraries.

Signed-off-by: Ian Jackson <ian.jackson@citrix.com>
[add trailing comma, fix typo, change bulleseye line]
Signed-off-by: Hans van Kranenburg <hans@knorrie.org>