[IA64] preparation for vga acceleration of VTI domain

[IA64] preparation for vga acceleration of VTI domain

This patch fixes several minor issues, as a prepare step to support VGA
acceleration for VTI domain:

- shared vram buffer needs to be mapped as WB in both sides, because
the shared vram is the true WB memory though VTI domain is told as UC
- Introduced a new pte_mem to indicate p2m entry containing valid mfn
when replace p2m entry. Pte_none doesn't work for VTI case, since IO
type is encoded into p2m entry which however doesn't contain valid mfn.

After above change, guest_physmap_add/remove_page is available to
be used by qemu to setup linear buffer for VTI domain.

Signed-off-by Kevin Tian <kevin.tian@intel.com>