evtchn: add FIFO-based event channel ABI

Add the event channel hypercall sub-ops and the definitions for the
shared data structures for the FIFO-based event channel ABI.

The design document for this new ABI is available here:

http://xenbits.xen.org/people/dvrabel/event-channels-F.pdf

In summary, events are reported using a per-domain shared event array
of event words.  Each event word has PENDING, LINKED and MASKED bits
and a LINK field for pointing to the next event in the event queue.

There are 16 event queues (with different priorities) per-VCPU.

Key advantages of this new ABI include:

- Support for over 100,000 events (2^17).
- 16 different event priorities.
- Improved fairness in event latency through the use of FIFOs.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>

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