HPCC-10470 Roxie Graphs are not getting merged correctly

The xgmml being returned from control:querystats was not layed out as expected
(an extra graph layer). While this did not cause any problems displaying the
graphs, it prevented the merge code from operating correctly.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #5304 from richardkchapman/roxie-stats

HPCC-10470 Roxie Graphs are not getting merged correctly

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>