Create a generic callback mechanism for Xen-bound event channels

Create a generic callback mechanism for  Xen-bound event channels

For event channels for which Xen is the consumer, there currently is
a single action. With this patch, we allow event channel creators to
specify a generic callback (or no callback). Because the expectation
is that there will be few callbacks, they are stored in a small table.

Signed-off-by: Adin Scannell <adin@scannell.ca>
Signed-off-by: Keir Fraser <keir@xen.org>
Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Committed-by: Tim Deegan <tim@xen.org>