OvmfPkg/VirtioPciDeviceDxe: implement IOMMU-like member functions

The patch implements the newly added IOMMU-like member functions by
respectively delegating the job to:

- VIRTIO_DEVICE_PROTOCOL.AllocateSharedPages () ->
    MemoryAllocationLib.AllocatePages()

- VIRTIO_DEVICE_PROTOCOL.FreeSharedPages () ->
    MemoryAllocationLib.FreePages ()

- VIRTIO_DEVICE_PROTOCOL.MapSharedBuffer () -> no-op

- VIRTIO_DEVICE_PROTOCOL.UnmapSharedBuffer () -> no-op

Suggested-by: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>

OvmfPkg/VirtioPciDeviceDxe/VirtioPciDevice.c

@@ -5,6 +5,7 @@
   Copyright (C) 2012, Red Hat, Inc.
   Copyright (c) 2012 - 2016, Intel Corporation. All rights reserved.<BR>
   Copyright (C) 2013, ARM Ltd.
+  Copyright (C) 2017, AMD Inc, All rights reserved.<BR>
 
   This program and the accompanying materials are licensed and made available
   under the terms and conditions of the BSD License which accompanies this
@@ -40,7 +41,11 @@ STATIC VIRTIO_DEVICE_PROTOCOL mDeviceProtocolTemplate = {
   VirtioPciGetDeviceStatus,             // GetDeviceStatus
   VirtioPciSetDeviceStatus,             // SetDeviceStatus
   VirtioPciDeviceWrite,                 // WriteDevice
-  VirtioPciDeviceRead                   // ReadDevice
+  VirtioPciDeviceRead,                  // ReadDevice
+  VirtioPciAllocateSharedPages,         // AllocateSharedPages
+  VirtioPciFreeSharedPages,             // FreeSharedPages
+  VirtioPciMapSharedBuffer,             // MapSharedBuffer
+  VirtioPciUnmapSharedBuffer,           // UnmapSharedBuffer
 };
 
 /**