hw/arm/virt: no ITS on older machine types

We should avoid exposing new hardware (through DT and ACPI) on older
machine types. This patch keeps 2.7 and older from changing, despite
the introduction of ITS support for 2.8.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Message-id: 1476117341-32690-3-git-send-email-drjones@redhat.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
