bus: simplify name handling

Simplify a bit the code by using g_strdup_printf() and store it in a
non-const value so casting is no longer needed, and ownership is
clearer.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

include/hw/qdev-core.h

@@ -224,7 +224,7 @@ typedef struct BusChild {
 struct BusState {
     Object obj;
     DeviceState *parent;
-    const char *name;
+    char *name;
     HotplugHandler *hotplug_handler;
     int max_index;
     bool realized;