machine: Make -machine opts properties of MachineState

Make machine's QemuOpts QOM properties of /machine. The properties
are automatically filled in. This opens the possibility to create
opts per machine rather than global.

Signed-off-by: Marcel Apfelbaum <marcel.a@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>