evtchn: block speculative out-of-bound accesses

evtchn: block speculative out-of-bound accesses

Guests can issue event channel interaction with guest specified data.
To avoid speculative out-of-bound accesses, we use the nospec macros,
or the domain_vcpu function. Where appropriate, we use the vcpu_id of
the seleceted vcpu instead of the parameter that can be influenced by
the guest, so that only one access needs to be protected.

This is part of the speculative hardening effort.

Signed-off-by: Norbert Manthey <nmanthey@amazon.de>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>