perf stat: Tell user about unsupported events in the list

Similar to perf-record, tell user about unsupported events
that will not be counted if invoked in verbose mode.

e.g.,

 $ perf stat -e dTLB-prefetch-misses -v -- sleep 1
 dTLB-prefetch-misses event is not supported by the kernel.
 dTLB-prefetch-misses: 0 0 0

 Performance counter stats for 'sleep 1':

     <not counted> dTLB-prefetch-misses

        1.001884783  seconds time elapsed

Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/1304114655-10600-1-git-send-email-dsahern@gmail.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>