cpu: Register VMStateDescription through CPUState

In comparison to DeviceClass::vmsd, CPU VMState is split in two,
"cpu_common" and "cpu", and uses cpu_index as instance_id instead of -1.
Therefore add a CPU-specific CPUClass::vmsd field.

Unlike the legacy CPUArchState registration, rather register CPUState.

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