s390x: Convert QEMUMachine to MachineClass

This converts s390-virtio and s390-ccw-virtio machines to QOM MachineClass.
This brings ability to add interfaces to the machine classes. The first
interface for addition will be NMI.

The patch is mechanical so no change in behavior is expected.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>