pc-dimm: Add description for device list.

The pc-dimm option presented on device list (by argument "-device \?")
is the unique option that don't have any information about it. This
patch adds a description for the pc-dimm device to help users to
identify it.
Signed-off-by: NPaulo Vital <paulo.vital@profitbricks.com>
Reviewed-by: NIgor Mammedov <imammedo@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/mem/pc-dimm.c

@@ -351,6 +351,7 @@ static void pc_dimm_class_init(ObjectClass *oc, void *data)
 
     dc->realize = pc_dimm_realize;
     dc->props = pc_dimm_properties;
+    dc->desc = "DIMM memory module";
 
     ddc->get_memory_region = pc_dimm_get_memory_region;
 }