plugin: Don't query for swap_events support directly

There is no need for the duplication, clutter already exports
this information.

https://bugzilla.gnome.org/show_bug.cgi?id=724870
perf: Restore shell after runs

Currently running the perf tool results into no wm running
afterwards making it hard for the user to get the results from a terminal
and generally does not make it easy for users to run it to gather numbers.

So restore the shell after the test has completed.

https://bugzilla.gnome.org/show_bug.cgi?id=724870