[VTD] Utilise the snoop control capability in shadow with VT-d code

[VTD] Utilise the snoop control capability in shadow with VT-d code

We compute the shadow PAT index in leaf page entries now as:
1) No VT-d assigned: let shadow PAT index as WB, handled already
in shadow code before.
2) direct assigned MMIO area: let shadow code compute the shadow
PAT with gMTRR=UC and gPAT value.
3) Snoop control enable: let shadow PAT index as WB.
4) Snoop control disable:  let shadow code compute the shadow
PAT with gMTRR and gPAT, handled already in shadow code before

Signed-off-by: Xin, Xiaohui <xiaohui.xin@intel.com>