x86/mm: Allow memevent responses to be signaled via the event channel

x86/mm: Allow memevent responses to be signaled via the event channel

Don't require a separate domctl to notify the memevent interface that an event
has occured.  This domctl can be taxing, particularly when you are scaling
events and paging to many domains across a single system.  Instead, we use the
existing event channel to signal when we place something in the ring (as per
normal ring operation).

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>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>