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>