hw/arm/virt: Add the virtio-iommu device tree mappings

Adds the "virtio,pci-iommu" node in the host bridge node and
the RID mapping, excluding the IOMMU RID.

This is done in the virtio-iommu-pci hotplug handler which
gets called only if no firmware is loaded or if -no-acpi is
passed on the command line. As non DT integration is
not yet supported by the kernel we must make sure we
are in DT mode. This limitation will be removed as soon
as the topology description feature gets supported.

Signed-off-by: Eric Auger <eric.auger@redhat.com>

Message-Id: <20200214132745.23392-10-eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>