x86/mm: Take care of domain reference for shared pages

x86/mm: Take care of domain reference for shared pages

Making a page sharable removes it from the previous owner's list. Making it
private adds it. These actions are similar to freeing or allocating a page.
Except that they were not minding the domain reference that is taken/dropped
when the first/last page is allocated/freed.

Without fixing this, a domain might remain zombie when destroyed if all its
pages are shared.

Signed-off-by: Andres Lagar-Cavilla <andres@lagarcavilla.org>
Acked-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>