pseries: Increase maximum CPUs to 256

The original pSeries machine was limited to 32 CPUs, more or less
arbitrarily.  Particularly when we get SMT KVM guests it will be
pretty easy to exceed this.  Therefore, raise the max number of CPUs
in a pseries machine guest to 256.

Signed-off-by: Anton Blanchard <anton@au1.ibm.com>
Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
