OvmfPkg/QemuBootOrderLib: add missing EFIAPI specifiers

OvmfPkg/QemuBootOrderLib: add missing EFIAPI specifiers

Public library APIs should be declared as EFIAPI.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Shannon Zhao <zhaoshenglong@huawei.com>
Cc: Xiang Zheng <xiang.zheng@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> # ArmVirtQemu
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Tested-by: Xiang Zheng <xiang.zheng@linaro.org>