Turbostat: Fix sched affinity mess

get_msr doesn't need to be run on the CPU targetted. However, when run
on another CPU, perfomances will decrease as task have to be scheduled
on another CPU. Migrating to another CPU during the setup (where only a
few get_msr are done) is probably not worth it. However, in the
get_counters function, the large number of get_msr query makes it
potentially important.

This patch also introduce the restoration of the scheduling affinity as
it was before the plugin invocation.