xsm/evtchn: never pretend to have successfully created a Xen event channel

xsm/evtchn: never pretend to have successfully created a Xen event channel

Xen event channels are not internal resources.  They still have one end in a
domain, and are created at the request of privileged domains.  This logic
which "successfully" creates a Xen event channel opens up undesirable failure
cases with ill-specified XSM policies.

If a domain is permitted to create ioreq servers or memevent listeners, but
not to create event channels, the ioreq/memevent creation will succeed but
attempting to bind the returned event channel will fail without any indication
of a permission error.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>