qdev: Check if unplug handler exists before calling it

A bus may have hotplugging enabled but not have the 'unplug'
callback defined, which would lead to a crash on trying to
unplug a device on the bus.

Fix by introducing an assert to check if the callback is valid.
Signed-off-by: NAmit Shah <amit.shah@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/qdev.c

@@ -258,6 +258,8 @@ int qdev_unplug(DeviceState *dev)
                    dev->parent_bus->name);
         return -1;
     }
+    assert(dev->info->unplug != NULL);
+
     return dev->info->unplug(dev);
 }