hw/acpi/aml-build: Make aml_buffer() definition consistent with the spec

According to ACPI spec, DefBuffer can take two parameters: BufferSize
and ByteList. Make it consistent with the spec. Uninitialized buffer
could be requested by passing ByteList as NULL to reserve space.

Signed-off-by: Shannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 1432522520-8068-15-git-send-email-zhaoshenglong@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
