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>
