OvmfPkg-EmuVariableFvbRuntimeDxe: Support Access To Memory Above 4G

OvmfPkg-EmuVariableFvbRuntimeDxe: Support Access To Memory Above 4G

In FvbInitialize Function,
PcdFlashNvStorageVariableBase64 PcdFlashNvStorageFtwWorkingBase
PcdFlashNvStorageFtwSpareBase will not exceed 0x100000000,
Due to truncation and variable type limitations.
That leads to the NV variable cannot be saved to the memory above 4G.

Modify as follows:
1.Remove the forced type conversion of UINT32.
2.Use UINT64 type variables.

Signed-off-by: xianglai li <lixianglai@loongson.cn>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>