public/x86: remove HVMMEM_mmio_write_dm from the public interface

public/x86: remove HVMMEM_mmio_write_dm from the public interface

HVMMEM_mmio_write_dm is removed for new xen interface versions, and
is replaced with type HVMMEM_unused. Attempts to set a page to this
type will return -EINVAL in xen after 4.7.0. And there will be no
pages with type p2m_mmio_write_dm, therefore HVMOP_get_mem_type will
never get the old type - HVMMEM_mmio_write_dm.

New approaches to write protect guest ram pages will be provided in
future patches.

Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>