hw/virtio/virtio-iommu-pci: Enforce the device is plugged on the root bus

hw/virtio/virtio-iommu-pci: Enforce the device is plugged on the root bus

In theory the virtio-iommu-pci could be plugged anywhere in the PCIe
topology and as long as the dt/acpi info are properly built this should
work. However at the moment we fail to do that because the
virtio-iommu-pci BDF is not computed at plug time and in that case
vms->virtio_iommu_bdf gets an incorrect value.

For instance if the virtio-iommu-pci is plugged onto a pcie root port
and the virtio-iommu protects a virtio-block-pci device the guest does
not boot.

So let's do not pretend we do support this case and fail the initialize()
if we detect the virtio-iommu-pci is plugged anywhere else than on the
root bus. Anyway this ability is not needed.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Message-Id: <20221012163448.121368-1-eric.auger@redhat.com>
Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Tested-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>