d/xen-utils-common.xen.init: disable oom killer for xenstored

d/xen-utils-common.xen.init: disable oom killer for xenstored

In case of oom killer terminating some process, we'd rather not see
xenstored go. Xenstored has an in-memory database, and when starting the
process again, it would be empty, which is very inconvenient. Xenstored
should already score quite low and have a fairly low memory footprint,
but according to the user report, it happened.

Closes: #961511
Suggested-by: Samuel Thibault <sthibault@debian.org>
Signed-off-by: Hans van Kranenburg <hans@knorrie.org>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>