xsm/flask: clean interdomain event channel hook

xsm/flask: clean interdomain event channel hook

Don't attempt to relabel the already-bound half of the event channel
pair created by an interdomain event channel. This relabeling also
performed an incorrect check that the destination domain is permitted
to create the reverse event channel, which may not be true if the
unbound channel was created by the domain builder (like the xenstore
channel).

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Committed-by: Keir Fraser <keir@xen.org>