Have a watch callback return 0 or 1 depending upon whether it would like to

Have a watch callback return 0 or 1 depending upon whether it would like to
continue to receive watches.  This means that it is not necessary to probe
around in the xswatch internals to unregister a watch.

Tidy up the hotplug watch handling, moving the nested function out to a
separate place (I don't think that this was a problem in the end, but it was
making me nervous as I was debugging the recent race condition, and I find it
clearer out of the class in any case.  Pass an integer code representing
hotplug status, once it has been parsed from the store, as there are now a few
different places we can detect failure, and it's cleaner to pass a code from
those places.

Small tidy up to XendDomain, removing the unused releaseDomain field.