Add a Xenctrl implementation of the events interface

Signed-off-by: David Scott <dave.scott@citrix.com>
Remove the anomalous xen-evtchn.unix.activations package

All our implementations will implement the full (threaded) signature
S.EVENTS, there should be no special-casing of activations.

Signed-off-by: David Scott <dave.scott@citrix.com>