cpu_{save,load} were no-ops, so de facto it is unmigratable and no
backwards compatibility to keep. Therefore mark the MicroBlazeCPU as
unmigratable at device level the QOM way and suppress "cpu_common"
VMState registration by dropping CPU_SAVE_VERSION.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Juan Quintela <quintela@redhat.com>