hw/core: Introduce QEMU machine as QOM object

The main functional change is to convert QEMUMachine into MachineClass
and QEMUMachineInitArgs into MachineState, instance of MachineClass.

As a first step, in order to make possible an incremental development,
both QEMUMachine and QEMUMachineInitArgs are being embedded into the
new types.

Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>