OvmfPkg/VirtHstiDxe: add varstore flash check

Detects qemu config issue: vars pflash is not in secure mode (write
access restricted to smm).  Applies to Q35 with SMM only.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Konstantin Kostiuk <kkostiuk@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>

OvmfPkg/VirtHstiDxe/VirtHstiDxe.inf

@@ -22,6 +22,7 @@
   VirtHstiDxe.c
   QemuPC.c
   QemuQ35.c
+  Flash.c
 
 [Packages]
   MdePkg/MdePkg.dec
@@ -46,5 +47,8 @@
 [FeaturePcd]
   gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire
 
+[Pcd]
+  gUefiOvmfPkgTokenSpaceGuid.PcdOvmfFlashNvStorageVariableBase
+
 [Depex]
   TRUE