spapr_iommu: Replace @instance_id with LIOBN for migration

SPAPR IOMMU is a bus-less device and therefore its only ID in
migration stream is an instance id which is not reliable ID
as it depends on the command line parameters order. Since
libvirt may change the order, we need something better than that.

This removes VMSD descriptor from the class definitiion and
registers it with @liobn as an intance ID to let the destination
side find the right device to receive migration data.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>