OvmfPkg/PlatformPei: silence X64 VS2015x86 warning

OvmfPkg/PlatformPei: silence X64 VS2015x86 warning

VS2015x86 reports the following warning for
"OvmfPkg/PlatformPei/MemDetect.c":

> MemDetect.c(357): error C2220: warning treated as error - no 'object'
>                   file generated
> MemDetect.c(357): warning C4244: '=': conversion from 'UINT64' to
>                   'UINT32', possible loss of data

LowerMemorySize is first assigned from GetSystemMemorySizeBelow4gb(),
which returns UINT32. Change the type of LowerMemorySize accordingly.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>