pmu: fix pmu vmstate subsection list

pmu: fix pmu vmstate subsection list

The subsection is not closed by a NULL marker so this can trigger
a segfault when the pmu vmstate is saved.

This can be easily shown with:

  $ ./qemu-system-ppc64  -dump-vmstate vmstate.json
  Segmentation fault (core dumped)

Fixes: d811d61fbc6c ("mac_newworld: add PMU device")
Cc: mark.cave-ayland@ilande.co.uk
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Cédric Le Goater <clg@kaod.org>