spapr: Add sPAPRMachineClass

spapr: Add sPAPRMachineClass

Currently although we have an sPAPRMachineState descended from MachineState
we don't have an sPAPRMAchineClass descended from MachineClass.  So far it
hasn't been needed, but several upcoming features are going to want it,
so this patch creates a stub implementation.

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