When xen-dir cannot find xen-utils, mention that this might be because
xen-utils-<RUNNING-XEN-VERSION> was already removed.

This is generally helpful, but it does not solve the `missing
xenconsoled' problems because 1. it only changes messages and
2. actually in the init script, the error message is currently
discarded anyway (!)

But, anyway, it is an improvemennt.

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