xen init script: Do nothing if running for wrong Xen package

See the big comment.  We think that this is responsible for various
bugs and, particularly, reports of mysteriously missing xenconsoled.

For example, this bug would mean that after a Xen version upgrade,
autoremoval of an obsolete xen-utils-V package would stop the running
xenconsoled.  This is obviously awkward to track down, and could occur
many weeks or months after the upgrade.

Closes: #851654
Signed-off-by: Ian Jackson <ian.jackson@citrix.com>
Acked-by: Hans van Kranenburg <hans@knorrie.org>