x86: add a new p2m type class - P2M_DISCARD_WRITE_TYPES

x86: add a new p2m type class - P2M_DISCARD_WRITE_TYPES

Currently, the P2M_RO_TYPES bears 2 meanings: one is
"_PAGE_RW bit is clear in their PTEs", and another is
to discard the write operations on these pages. This
patch adds a p2m type class, P2M_DISCARD_WRITE_TYPES,
to bear the second meaning, so we can use this type
class instead of the P2M_RO_TYPES, to decide if a write
operation is to be ignored.

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Reviewed-by: Jan Beulich <jbeulich@suse.com>