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/Library/BaseMemEncryptSevLib/X64/VirtualMemory.c

@@ -2,7 +2,7 @@
 
   Virtual Memory Management Services to set or clear the memory encryption bit
 
-  Copyright (c) 2006 - 2016, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2006 - 2018, Intel Corporation. All rights reserved.<BR>
   Copyright (c) 2017, AMD Incorporated. All rights reserved.<BR>
 
   This program and the accompanying materials are licensed and made available
@@ -596,6 +596,11 @@ SetMemoryEncDec (
   BOOLEAN                        IsWpEnabled;
   RETURN_STATUS                  Status;
 
+  //
+  // Set PageMapLevel4Entry to suppress incorrect compiler/analyzer warnings.
+  //
+  PageMapLevel4Entry = NULL;
+
   DEBUG ((
     DEBUG_VERBOSE,
     "%a:%a: Cr3Base=0x%Lx Physical=0x%Lx Length=0x%Lx Mode=%a CacheFlush=%u\n",