spapr: Add kvm-type property

The kvm-type machine option was left out when MachineState was
introduced, preventing the kvm-type option from being used. Add the
missing property to the sPAPR machine class, so it can be used.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Tested-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
