Remove io_index argument from cpu_register_io_memory()

The parameter is always zero except when registering the three internal
io regions (ROM, unassigned, notdirty).  Remove the parameter to reduce
the API's power, thus facilitating future change.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/hw/hpet.c b/hw/hpet.c
index 29db325..e0be486 100644
--- a/hw/hpet.c
+++ b/hw/hpet.c
@@ -582,7 +582,7 @@
     register_savevm("hpet", -1, 1, hpet_save, hpet_load, s);
     qemu_register_reset(hpet_reset, 0, s);
     /* HPET Area */
-    iomemtype = cpu_register_io_memory(0, hpet_ram_read,
+    iomemtype = cpu_register_io_memory(hpet_ram_read,
                                        hpet_ram_write, s);
     cpu_register_physical_memory(HPET_BASE, 0x400, iomemtype);
 }