x86/mm/p2m: Make p2m interfaces take struct domain arguments.

x86/mm/p2m: Make p2m interfaces take struct domain arguments.

As part of the nested HVM patch series, many p2m functions were changed
to take pointers to p2m tables rather than to domains.  This patch
reverses that for almost all of them, which:
 - gets rid of a lot of "p2m_get_hostp2m(d)" in code which really
   shouldn't have to know anything about how gfns become mfns.
 - ties sharing and paging interfaces to a domain, which is
   what they actually act on, rather than a particular p2m table.

In developing this patch it became clear that memory-sharing and nested
HVM are unlikely to work well together.  I haven't tried to fix that
here beyond adding some assertions around suspect paths (as this patch
is big enough with just the interface changes)

Signed-off-by: Tim Deegan <Tim.Deegan@citrix.com>