XML state system: Add "eventName" to the current stateattribute types.

XML state system: Add "eventName" to the current stateattribute types.

We already have the "eventName" as a type for statevalue, but sometimes we need
it as a valid stateattribute type as well, e.g., in counting the number of each
event type.

Change-Id: Ie699221d71cee1686c6865bcffc3b38a991a0731
Signed-off-by: Naser Ezzati <n.ezzati@polymtl.ca>
Reviewed-on: https://git.eclipse.org/r/36059
Tested-by: Hudson CI
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>