hw/arm: Restrict APEI tables generation to the 'virt' machine

hw/arm: Restrict APEI tables generation to the 'virt' machine

While APEI is a generic ACPI feature (usable by X86 and ARM64), only
the 'virt' machine uses it, by enabling the RAS Virtualization. See
commit 2afa8c8519: "hw/arm/virt: Introduce a RAS machine option").

Restrict the APEI tables generation code to the single user: the virt
machine. If another machine wants to use it, it simply has to 'select
ACPI_APEI' in its Kconfig.

Fixes: aa16508f1d ("ACPI: Build related register address fields via hardware error fw_cfg blob")
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Dongjiu Geng <gengdongjiu@huawei.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20201008161414.2672569-1-philmd@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>