ide: Fix bootindex for bus_id > 9

We identify devices by their Open Firmware device paths.  The encoding
of bus numbers is incorrect: idebus_get_fw_dev_path() formats them in
decimal, while SeaBIOS uses hexadecimal.  With bus number > 9, SeaBIOS
will miss the bootindex (lucky case), or apply it to another device
(unlucky case).

Bug can't bite right now: ich9-ahci has six ports, and the sysbus-ahci
created by Calxeda Highbank has just one.

Fix it anyway, by changing %d to %x.

I couldn't find an Open Firmware spec covering this.  For what it's
worth, OVMF agrees with SeaBIOS.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/ide/qdev.c

@@ -59,7 +59,7 @@ static char *idebus_get_fw_dev_path(DeviceState *dev)
 {
     char path[30];
 
-    snprintf(path, sizeof(path), "%s@%d", qdev_fw_name(dev),
+    snprintf(path, sizeof(path), "%s@%x", qdev_fw_name(dev),
              ((IDEBus*)dev->parent_bus)->bus_id);
 
     return g_strdup(path);