hw/arm/virt-acpi-build: Only expose flash on older machine types

The flash device is exclusively for the host-controlled firmware, so
we should not expose it to the OS. Exposing it risks the OS messing
with it, which could break firmware runtime services and surprise the
OS when all its changes disappear after reboot.

As firmware needs the device and uses DT, we leave the device exposed
there. It's up to firmware to remove the nodes from DT before sending
it on to the OS. However, there's no need to force firmware to remove
tables from ACPI (which it doesn't know how to do anyway), so we
simply don't add the tables in the first place. But, as we've been
adding the tables for quite some time and don't want to change the
default hardware exposed to versioned machines, then we only stop
exposing the flash device tables for 5.1 and later machine types.

Suggested-by: Ard Biesheuvel <ard.biesheuvel@arm.com>
Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-id: 20200629140938.17566-4-drjones@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

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