pc: Don't use QEMUMachine anymore

Now that we have a DEFINE_PC_MACHINE helper macro that just requires an
initialization function, it is trivial to convert them to register a QOM
machine class directly, instead of using QEMUMachine.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>