iotests: 087: Don't attach test image twice

The test scenario doesn't require the same image, instead it focuses on
the duplicated node-name, so use null-co to avoid locking conflict.
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/087

@@ -82,8 +82,7 @@ run_qemu -drive driver=$IMGFMT,id=disk,node-name=test-node,file="$TEST_IMG" <<EO
       "driver": "$IMGFMT",
       "node-name": "disk",
       "file": {
-          "driver": "file",
-          "filename": "$TEST_IMG"
+          "driver": "null-co"
       }
     }
   }
@@ -92,8 +91,7 @@ run_qemu -drive driver=$IMGFMT,id=disk,node-name=test-node,file="$TEST_IMG" <<EO
       "driver": "$IMGFMT",
       "node-name": "test-node",
       "file": {
-          "driver": "file",
-          "filename": "$TEST_IMG"
+          "driver": "null-co"
       }
     }
   }