tools/power turbostat: make fewer systems calls

BugLink: http://bugs.launchpad.net/bugs/1591802



skip the open(2)/close(2) on each msr read
by keeping the /dev/cpu/*/msr files open.

The remaining read(2) is generally far fewer cycles
than the removed open(2) system call.

Signed-off-by: Len Brown <len.brown@intel.com>
(cherry picked from commit 36229897)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Acked-by: Kamal Mostafa <kamal@canonical.com>