x86/mm: When removing/adding a page from/to the physmap, keep in mind it could be...

x86/mm: When removing/adding a page from/to the physmap, keep in mind it could be shared

When removing the m2p mapping it is unconditionally set to invalid, which
breaks sharing.

When adding to the physmap, if the previous holder of that entry is a shared
page, we unshare to default to normal case handling.

And, we cannot add a shared page directly to the physmap. Proper interfaces
must be employed, otherwise book-keeping goes awry.

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