ppc: Update cpu_model in MachineState

Keep cpu_model field in MachineState uptodate so that it can be used
from the CPU hotplug path.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>