x86/shadow: tidy up fragmentary page lists in multi-page shadows

x86/shadow: tidy up fragmentary page lists in multi-page shadows

Multi-page shadows are linked together using the 'list' field.  When
those shadows are in the pinned list, the list fragments are spliced
into the pinned list; otherwise they have no associated list head.

Rework the code that handles these fragments to use the page_list
interface rather than manipulating the fields directly.  This makes
the code cleaner, and allows the 'list' field to be either the
compact pdx form or a normal list_entry.

Signed-off-by: Tim Deegan <tim@xen.org>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Introduce sh_terminate_list() and make it use LIST_POISON*.

Move helper array of shadow_size() into common.c.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Tim Deegan <tim@xen.org>