machine: Set MachineClass::name automatically

Now all TYPE_MACHINE subclasses use MACHINE_TYPE_NAME to generate the
class name. So instead of requiring each subclass to set
MachineClass::name manually, we can now set it automatically at the
TYPE_MACHINE class_base_init() function.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>
[AF/ehabkost: Updated for s390-ccw machines]
[AF: Cleanup of intermediate virt and vexpress name handling]
Signed-off-by: Andreas Färber <afaerber@suse.de>