hw/ipmi: Move vmsd registration to device class

hw/ipmi: Move vmsd registration to device class

It's the right way to do it now, and it handles multiple instances
properly.  I need multiple instances for some testing I'm doing so this
is the right thing to do.

Tested by doing:

   (qemu) migrate -d exec:cat>filen.mig

before and after the fix, then:

   scripts/analyze-migration.py -d desc -f file1.mig >file1.json
   scripts/analyze-migration.py -d desc -f file2.mig >file2.json
   diff file1.json file2.json

with no differences.

Signed-off-by: Corey Minyard <corey@minyard.net>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Fabiano Rosas <farosas@suse.de>