test: xml2xml: Print full filenames if xml2xml test fails

To simplify looking for a problem instrument the XML comparator function
with possibility to print the filename of the failed/expected XML
output.

This is necessary as the VIR_TEST_DIFFERENT macro possibly tests two XML
files for the inactive/active state and the resulting error may not be
obvious.

tests/qemuxml2xmltest.c

@@ -50,7 +50,7 @@ testCompareXMLToXMLFiles(const char *inxml, const char *outxml, bool live)
         goto fail;
 
     if (STRNEQ(outXmlData, actual)) {
-        virtTestDifference(stderr, outXmlData, actual);
+        virtTestDifferenceFull(stderr, outXmlData, outxml, actual, inxml);
         goto fail;
     }