hw/arm/virt: Reject instantiation of multiple IOMMUs

hw/arm/virt: Reject instantiation of multiple IOMMUs

We do not support instantiating multiple IOMMUs. Before adding a
virtio-iommu, check that no other IOMMU is present. This will detect
both "iommu=smmuv3" machine parameter and another virtio-iommu instance.

Fixes: 70e89132c9 ("hw/arm/virt: Add the virtio-iommu device tree mappings")
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Message-id: 20211210170415.583179-4-jean-philippe@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>