M2P translation cannot be handled through flat table with only one slot per MFN

M2P translation cannot be handled through flat table with only one slot per MFN
when an MFN is shared. However, all existing calls can either infer the GFN (for
example p2m table destructor) or will not need to know GFN for shared pages.
This patch identifies and fixes all the M2P accessors, either by removing the
translation altogether or by making the relevant modifications. Shared MFNs have
a special value of SHARED_M2P_ENTRY stored in their M2P table slot.

Signed-off-by: Grzegorz Milos <Grzegorz.Milos@citrix.com>