hw/boards: Convert current_machine to MachineState

In order to allow attaching machine options to a machine instance,
current_machine is converted into MachineState.
As a first step of deprecating QEMUMachine, some of the functions
were modified to return MachineClass.

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