evtchn/fifo: use stable fields when recording "last queue" information

evtchn/fifo: use stable fields when recording "last queue" information

Both evtchn->priority and evtchn->notify_vcpu_id could change behind the
back of evtchn_fifo_set_pending(), as for it - in the case of
interdomain channels - only the remote side's per-channel lock is held.
Neither the queue's priority nor the vCPU's vcpu_id fields have similar
properties, so they seem better suited for the purpose. In particular
they reflect the respective evtchn fields' values at the time they were
used to determine queue and vCPU.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Paul Durrant <paul@xen.org>