ArmVirtXen: Link VarCheckUefiLib NULL class library instance

ArmVirtXen: Link VarCheckUefiLib NULL class library instance

This patch is not related directly to
REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1323
Merge EmuVariable and Real variable driver.

It just makes ArmVirtXen be aligned with ArmVirtQemuXXX to
link VarCheckUefiLib NULL class library instance.

Suggested-by: Laszlo Ersek <lersek@redhat.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Julien Grall <julien.grall@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Julien Grall <julien.grall@arm.com>
Acked-by: Julien Grall <julien.grall@arm.com>