This patch defines a new P2M type used for sharable/shared pages. It also

This patch defines a new P2M type used for sharable/shared pages. It also
implements the basic functions to nominate GFNs for sharing, and to break
sharing (either by making page 'private' or creating private copy),
mem_sharing_nominate_page() and mem_sharing_unshare_page() respectively. Note
pages cannot be shared yet, because there is no efficient way to find all GFNs
mapping to the two MFNs scheduled for sharing.

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