blktrace: load improvements

- Set the state explicitly to SETTING_UP when loading the
  traces. This could take a while, so tell the status thread
  that we are setting up so it knows how to display us
  correctly.

- Fix a bug with multiple devices where we didn't cache the
  major/minor properly. This made blktrace spend tons of CPU
  cycles looking up /sys.

- Handle multiple devices correctly when adding files.

Signed-off-by: Jens Axboe <axboe@fb.com>