x86/mm: Update mem sharing interface to (re)allow sharing of grants

x86/mm: Update mem sharing interface to (re)allow sharing of grants

Previously, the mem sharing code would return an opaque handle to index shared
pages (and nominees) in its global hash table.  By removing the hash table, the
new interfaces requires a gfn and a version. However, when sharing grants, the
caller provides a grant ref and a version. Update interface to handle this
case.

The use case for grant sharing is when sharing from within a backend (e.g.
memshr + blktap2), in which case the backend is only exposed to grant
references.

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