xen init script: silently exit status 0 if not running under xen

This script should not complain at all if we are not running under
Xen.  Check this right at the start.

This will enable improvements to the following code, which will no
longer have to deal with the `not running under Xen' case.

Signed-off-by: Ian Jackson <ian.jackson@citrix.com>
Acked-by: Hans van Kranenburg <hans@knorrie.org>