xml: bug 497959 add the possibility to update the current state

xml: bug 497959 add the possibility to update the current state

This adds an 'update' attribute to state values, such that it uses the
updateOngoingState method of the state system to change the value of the
current state instead of modifying it.

Change-Id: I44b80c1801b9dbbb4f858ba7e3c05434d84f9003
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/77797
Reviewed-by: Hudson CI
Reviewed-by: Jean-Christian Kouame <jean-christian.kouame@ericsson.com>
Tested-by: Jean-Christian Kouame <jean-christian.kouame@ericsson.com>