Linux/xencommons: Use oxenstored by default when available

Linux/xencommons: Use oxenstored by default when available

oxenstored is an ocaml implementation of the xenstore daemon. It is faster,
more scalable and more reliable than the C xenstored.

In particular the transaction model in oxenstored does not involve taking a
complete copy of the database and aborting on any (even non-conflicting) other
change.

There is a paper on the design and implementation of oxenstored at
http://gazagnaire.org/pub/GH09.pdf which includes a performance evaluation and
comparison with the C daemon etc.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>