The intel-iommu device has existed since QEMU 2.2.0, but
it was only possible to create it with -device since
QEMU 2.7.0, thanks to:

  commit 621d983a1f9051f4cfc3f402569b46b77d8449fc
  Author: Marcel Apfelbaum <marcel@redhat.com>
  Date:   Mon Jun 27 18:38:34 2016 +0300

    hw/iommu: enable iommu with -device

    Use the standard '-device intel-iommu' to create the IOMMU device.
    The legacy '-machine,iommu=on' can still be used.

The libvirt capability check & command line formatting code
is thus broken for all QEMU versions 2.2.0 -> 2.6.0 inclusive.

This fixes it to use iommu=on instead.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>