i8257: Make device "i8257" unavailable with -device

The ISA DMA controller needs to be wired up to the ISA bus by
isa_bus_dma() to actually work.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Message-Id: <1472660151-19517-1-git-send-email-armbru@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/dma/i8257.c

@@ -600,6 +600,8 @@ static void i8257_class_init(ObjectClass *klass, void *data)
     idc->release_DREQ = i8257_dma_release_DREQ;
     idc->schedule = i8257_dma_schedule;
     idc->register_channel = i8257_dma_register_channel;
+    /* Reason: needs to be wired up by isa_bus_dma() to work */
+    dc->cannot_instantiate_with_device_add_yet = true;
 }
 
 static const TypeInfo i8257_info = {