Update memshr API and tools

Update memshr API and tools

This patch is the folded version of API updates, along with the associated tool
changes to ensure that the build is always consistent.

API updates:
- The source domain in the sharing calls is no longer assumed to be dom0.
- 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 handle becomes a version, to avoid sharing a stale version of a
  page. Thus, libxc wrappers and tools need to be updated to recall the share
  functions with the information needed to fetch the page (which they readily
  have).

Tool updates:
The only (in-tree, that we know of) consumer of the mem sharing API is the
memshr tool. This is updated to use the new API.

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