Fix regression with XML output.

[strongly-typed]

Copy-paste error from 9edcc44.

Spotted with cd test && ./generate-xml.sh test-frames

--- test-frames/manual_frame2.xml   2016-05-09 00:19:44.000000000 +0200
+++ test-frames/manual_frame2.xml.new   2016-05-09 01:09:09.000000000 +0200
@@ -17,7 +17,7 @@
     <DataRecord id="1">
         <Function>Actual value</Function>
         <Unit>reserved but historic</Unit>
-        <Value>135</Value>
+        <Value>1</Value>
     </DataRecord>

 </MBusData>