There was no CPU_SAVE_VERSION defined, so neither "cpu_common" VMState
nor cpu_{save,load}() were registered. Their implementation was no-op.
Therefore there is no backwards compatibility to keep, so mark XtensaCPU
as unmigratable at device level.

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