OvmfPkg: Check for QemuFwCfg availability before accessing it

There are few places in the codebase assuming QemuFwCfg will be present
and supported, which can cause some issues when trying to rely on the
QemuFwCfgLibNull implementation of QemuFwCfgLib.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
Acked-by: Jiewen Yao <jiewen.yao@intel.com>

OvmfPkg/Library/PlatformInitLib/Platform.c

@@ -410,14 +410,17 @@ PlatformMaxCpuCountInitialization (
   IN OUT EFI_HOB_PLATFORM_INFO  *PlatformInfoHob
   )
 {
-  UINT16  BootCpuCount;
+  UINT16  BootCpuCount = 0;
   UINT32  MaxCpuCount;
 
   //
   // Try to fetch the boot CPU count.
   //
-  QemuFwCfgSelectItem (QemuFwCfgItemSmpCpuCount);
-  BootCpuCount = QemuFwCfgRead16 ();
+  if (QemuFwCfgIsAvailable ()) {
+    QemuFwCfgSelectItem (QemuFwCfgItemSmpCpuCount);
+    BootCpuCount = QemuFwCfgRead16 ();
+  }
+
   if (BootCpuCount == 0) {
     //
     // QEMU doesn't report the boot CPU count. (BootCpuCount == 0) will let