hw/sparc64: set iommu_platform=on for virtio devices attached to the sun4u machine

hw/sparc64: set iommu_platform=on for virtio devices attached to the sun4u machine

The sun4u machine has an IOMMU and therefore it is possible to program it such
that the virtio-device IOVA does not map directly to the CPU physical address.

This is not a problem with Linux which always maps the IOVA directly to the CPU
physical address, however it is required for the NetBSD virtio driver where this
is not the case.

Set the sun4u machine defaults for all virtio devices so that disable-legacy=on
and iommu_platform=on to ensure a default configuration will allow virtio
devices to function correctly on both Linux and NetBSD.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20240418205730.31396-1-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>