vl: Use MachineClass instead of global QEMUMachine list

The machine registration flow is refactored to use the QOM functionality.
Instead of linking the machines into a list, each machine has a type
and the types can be traversed in the QOM way.

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>