Use a reserved pfn in the guest address space to store mem event rings

Use a reserved pfn in the guest address space to store mem event rings

This solves a long-standing issue in which the pages backing these rings were
pages belonging to dom0 user-space processes. Thus, if the process would die
unexpectedly, Xen would keep posting events to a page now belonging to some
other process.

We update all API-consumers in tree (xenpaging and xen-access).

This is an API/ABI change, so please speak up if it breaks your accumptions.

The patch touches tools, hypervisor x86/hvm bits, and hypervisor x86/mm bits.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Tim Deegan <tim@xen.org>