qemu: block: Always set discard for storage nodes

Enabling discard for the storage node allows the format drivers to
discard snapshots and other things, while configuration of the format
layer actually decides whether to actually discard data on request from
the host.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_block.c

@@ -1163,7 +1163,10 @@ qemuBlockStorageSourceGetBackendProps(virStorageSourcePtr src,
         if (qemuBlockStorageSourceGetBlockdevGetCacheProps(src, fileprops) < 0)
             goto cleanup;
 
-        if (virJSONValueObjectAdd(fileprops, "b:read-only", src->readonly, NULL) < 0)
+        if (virJSONValueObjectAdd(fileprops,
+                                  "b:read-only", src->readonly,
+                                  "s:discard", "unmap",
+                                  NULL) < 0)
             goto cleanup;
     }