ctf: Make packet descriptor information available to event

ctf: Make packet descriptor information available to event

This allows the event to know what the packet context is for
its parent packet. This is especially useful if one puts non-
LTTng information in the packet such as the source gpu.

Change-Id: I21ff283c1032e5a3157408b304b2611d1113d4f7
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/61816
Reviewed-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>