ppc/spapr: Fix MAX_CPUS to 255

MAX_CPUS 256 is inconsistent with qemu supporting upto 255 cpus. This
MAX_CPUS number was percolated back to "virsh capabilities" with wrong
max_cpus.

Signed-off-by: Nikunj A Dadhania <nikunj@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>