Fix qvm-trim-template block attach handling

qvm-trim-template manually assemble device dict, fix it to include
also 'desc' field, as it is now checked.
Fixes ba4dec5 "qvm-block: fix checking attached status of disk image files"

(cherry picked from commit 9d1fea4)

qvm-tools/qvm-trim-template

@@ -134,6 +134,7 @@ def main():
                                 'vm': 'dom0',
                                 'device': tvm.root_img,
                                 'mode': 'w',
+                                'desc': '',
                             },
                             mode='w',
                             frontend='xvdi')
@@ -168,4 +169,4 @@ def main():
 
 
 if __name__ == "__main__":
-    main()
+    main()