x86/mem_sharing: modularize reverse map for shared frames

x86/mem_sharing: modularize reverse map for shared frames

Each shared frame maintains a reverse map of <domain, gfn> tuples, so we know
which tuples this shared frame is backing. This is useful for auditing and
sanity-checking, and necessary to update all relevant p2m entries when sharing.

The reverse map is maintained as a doubly linked list, but the interface is
open-coded throughout the mem_sharing.c subsystem. Bury it inside a level of
abstraction, so it can later support different (more scalable) rmap
implementations.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>