intel_iommu: Correct caching-mode error message

intel_iommu: Correct caching-mode error message

If we try to use the intel-iommu device with vfio-pci devices without
caching mode enabled, we're told:

  qemu-system-x86_64: We need to set caching-mode=1 for intel-iommu to enable
  device assignment with IOMMU protection.

But to enable caching mode, the option is actually "caching-mode=on".

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Message-Id: <155364147432.16467.15898335025013220939.stgit@gimli.home>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Alex Williamson &lt;<a href="mailto:alex.williamson@redhat.com" target="_blank" rel="noreferrer">alex.williamson@redhat.com</a>&gt;<br>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>