This patch defines a new PGT type called PGT_shared_page and a new synthetic

This patch defines a new PGT type called PGT_shared_page and a new synthetic
domain called 'dom_cow'. In order to share a page, the type needs to be changed
to PGT_shared_page and the owner to dom_dow. Only pages with PGT_none, and no
type count are allowed to become sharable. Conversly, sharable pages can only be
made 'private' if type count equals one. page_make_sharable() and
page_make_private() handle these transitions.

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