This patch establishes a new abstraction of sharing handles (encoded as a 64bit

This patch establishes a new abstraction of sharing handles (encoded as a 64bit
int), each corresponding to a single sharable pages. Externally all sharing related
operations (e.g. nominate/share) will use sharing handles, thus solving a lot of
consistency problems (like: is this sharable page still the same sharable page
as before).
Internally, sharing handles can be translated to the MFNs (using a newly created
hashtable), and then for each MFNs a doubly linked list of GFNs translating to
this MFN is maintained. Finally, sharing handle is stored in page_info strucutre
for each sharable MFN.
All this allows to share and unshare pages efficiently. However, at the moment a
single lock is used to protect the sharing handle hash table. For scalability
reasons, the locking needs to be made more granular.

Signed-off-by: Grzegorz Milos <Grzegorz.Milos@citrix.com>