OvmfPkg:Fix VS2012 build failure

Initialize local variable to suppress warning C4701/C4703:
potentially uninitialized local variable/pointer variable.

1.In VirtualMemory.c:
Read of "PageMapLevel4Entry" in SetMemoryEncDe() is only
reached when "PageMapLevel4Entry" is got correctly.

2.In VirtioBlk.c:
Reads (dereferences) of "BufferMapping" and "BufferDeviceAddress"
in SynchronousRequest() are only reached if "BufferSize > 0" *and*
we map the data buffer successfully.

3.In VirtioScsi.c:
Reads (dereferences) of "InDataMapping" and "InDataDeviceAddress",
in VirtioScsiPassThru() are only reached if
"Packet->InTransferLength > 0" on input, *and* we map the
input buffer successfully. The similar reason for "OutDataMapping"
and "OutDataDeviceAddress".

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

OvmfPkg/VirtioScsiDxe/VirtioScsi.c

@@ -26,7 +26,7 @@
     unreasonable for now.
 
   Copyright (C) 2012, Red Hat, Inc.
-  Copyright (c) 2012 - 2014, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2012 - 2018, Intel Corporation. All rights reserved.<BR>
   Copyright (c) 2017, AMD Inc, All rights reserved.<BR>
 
   This program and the accompanying materials are licensed and made available
@@ -455,6 +455,15 @@ VirtioScsiPassThru (
   UINTN                     InDataNumPages;
   BOOLEAN                   OutDataBufferIsMapped;
 
+  //
+  // Set InDataMapping,OutDataMapping,InDataDeviceAddress and OutDataDeviceAddress to
+  // suppress incorrect compiler/analyzer warnings.
+  //
+  InDataMapping        = NULL;
+  OutDataMapping       = NULL;
+  InDataDeviceAddress  = 0;
+  OutDataDeviceAddress = 0;
+
   ZeroMem ((VOID*) &Request, sizeof (Request));
 
   Dev = VIRTIO_SCSI_FROM_PASS_THRU (This);