OvmfPkg: detect Xen earlier

This is useful for initializing memory map.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14943 6f19259b-4bc3-4df7-8a09-765794883524

OvmfPkg/PlatformPei/Platform.h

@@ -69,6 +69,11 @@ PeiFvInitialization (
 
 EFI_STATUS
 InitializeXen (
+  UINT32 XenLeaf
+  );
+
+UINT32
+XenDetect (
   VOID
   );