relocate p2m_access_t into common and swap the order

relocate p2m_access_t into common and swap the order

We swap the order of the enum of types n ... rwx, as to have rwx at 0, which is
the default setting when mem_access is not in use. This has performance benefits for
non-memaccess paths, as now comparison is to 0 when checking if memaccess is in use,
which is often faster.

We fix one location in nested_hap where the order of the enum made a difference.

Signed-off-by: Tamas K Lengyel <tklengyel@sec.in.tum.de>
Acked-by: Tim Deegan <tim@xen.org>