Tmf: Fix a bug in CFV and Xml time graph views where entries have no parent

Tmf: Fix a bug in CFV and Xml time graph views where entries have no parent

If a child entry starts before the parent entry, like it happens at the
beginning of a trace, then in time graph views, they are not associated with
the parent.

Change-Id: I3fb72049248c13aecdc41e88420eaf4dfae24911
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/35342
Tested-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>