conf: Introduce virtio options for virtio memory models

conf: Introduce virtio options for virtio memory models

Both virtio-mem and virtio-pmem memory models are virtio devices
and as such support setting various virtio knobs (iommu, ats,
packed, page_per_vq) common to other virtio devices.

Introduce <driver/> element as a child to <memory/> element, just
like we do for other virtio devices, where aforementioned knobs
live.

NB, this is without docs changes, since we do not document which
virtio devices support these knobs and each one is already
documented.

Also, the virtio-options.xml test needed some additional
adjustment (apart from adding virtio-mem device) to enable memory
hotplug.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>