nodedev: make iommuGroup optional for mdevs

nodedev: make iommuGroup optional for mdevs

When parsing a nodedev xml file, the iommuGroup element should be
optional. This element should be read-only and is determined by the
device driver. While this is a change to existing behavior, it doesn't
break backwards-compatibility because it makes the parser less strict.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>