hw/arm/virt: Default to two-stage SMMU from virt-9.2

hw/arm/virt: Default to two-stage SMMU from virt-9.2

Now that our SMMU model supports enabling both stages of translation
at once, we can enable this in the virt board.  This is no change in
behaviour for guests, because if they simply ignore stage 2 and never
configure it then it has no effect.  For the usual backwards
compatibility reasons we enable this only for machine types starting
with 9.2.

(Note that the SMMU is disabled by default on the virt board and is
only created if the user passes the 'iommu=smmuv3' machine option.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20240816161350.3706332-4-peter.maydell@linaro.org