OvmfPkg: QemuFlashFvbServicesRuntimeDxe: adhere to -D SMM_REQUIRE

OvmfPkg: QemuFlashFvbServicesRuntimeDxe: adhere to -D SMM_REQUIRE

When the user requires "security" by passing -D SMM_REQUIRE, and
consequently by setting PcdSmmSmramRequire, enforce flash-based variables.

Furthermore, add two ASSERT()s to catch if the wrong module were pulled
into the build.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19063 6f19259b-4bc3-4df7-8a09-765794883524