Convert machine registration to use module init functions

This cleans up quite a lot of #ifdefs, extern variables, and other ugliness.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/hw/ppc_oldworld.c b/hw/ppc_oldworld.c
index a822bfe..c6ec635 100644
--- a/hw/ppc_oldworld.c
+++ b/hw/ppc_oldworld.c
@@ -381,9 +381,16 @@
     qemu_register_boot_set(fw_cfg_boot_set, fw_cfg);
 }
 
-QEMUMachine heathrow_machine = {
+static QEMUMachine heathrow_machine = {
     .name = "g3beige",
     .desc = "Heathrow based PowerMAC",
     .init = ppc_heathrow_init,
     .max_cpus = MAX_CPUS,
 };
+
+static void heathrow_machine_init(void)
+{
+    qemu_register_machine(&heathrow_machine);
+}
+
+machine_init(heathrow_machine_init);