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/testutils.h

@@ -64,6 +64,11 @@ void virtTestClearCommandPath(char *cmdset);
 int virtTestDifference(FILE *stream,
                        const char *expect,
                        const char *actual);
+int virtTestDifferenceFull(FILE *stream,
+                           const char *expect,
+                           const char *expectName,
+                           const char *actual,
+                           const char *actualName);
 int virtTestDifferenceBin(FILE *stream,
                           const char *expect,
                           const char *actual,