tmf: Bug 477794: Persist trace group root during remote fetching

tmf: Bug 477794: Persist trace group root during remote fetching

This will improve the import feature of LTTng Control the performance
because it doesn't scan the parent folder for the relevant traces of
a session.

Change-Id: I07739c7e229c2026cb8393a19061152a51678dde
Signed-off-by: Bernd Hufmann <Bernd.Hufmann@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/56224
Reviewed-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>