xen/events: add struct evtchn_ops for the low-level port operations

evtchn_ops contains the low-level operations that access the shared
data structures.  This allows alternate ABIs to be supported.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>