hw/arm/virt: Enable dynamic generation of ACPI v5.1 tables

Initialize VirtGuestInfoState and register a machine_init_done notify to
call virt_acpi_build().

Signed-off-by: Shannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Message-id: 1432522520-8068-25-git-send-email-zhaoshenglong@huawei.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
