qdev: Fix bus dependency of DeviceState::hotpluggable getter Commit 1a37eca107cece3ed454bae29eef0bd1fac4a244 (qdev: add "hotpluggable" property to Device) added a property "hotpluggable" to each device, with its getter accessing parent_bus->allow_hotplug. Add a NULL check. Cc: Igor Mammedov <imammedo@redhat.com> Signed-off-by: Andreas Färber <afaerber@suse.de>