libxl: timeouts: Record deregistration when one occurs

libxl: timeouts: Record deregistration when one occurs

When a timeout has occurred, it is deregistered.  However, we failed
to record this fact by updating etime->func.  As a result,
libxl__ev_time_isregistered would say `true' for a timeout which has
already happened.

It is necessary to clear etime->func before the callback, because the
callback might want to reinstate the timeout, or might free the etime
(or its containing struct) entirely.

The results are that we might try to have the timeout occur again
(causing problems for the call site), and/or corrupt the timeout list.

This fixes the timedereg event system unit test.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Jim Fehlig <jfehlig@suse.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>