x86/mm: remove 'p2m_guest' lookup type.

x86/mm: remove 'p2m_guest' lookup type.

It was neither consistently used by callers nor correctly handled by the
lookup code.  Instead, treat any lookup that might allocate or unshare
memory as a 'guest' lookup for the purposes of:
 - detecting the highest pod gfn populated; and
 - crashing the guest on access to a broken page
which were the only things this was used for.

Signed-off-by: Tim Deegan <tim@xen.org>
Committed-by: Tim Deegan <tim@xen.org>