Use blkio.throttle.* stats when CFQ is not in use

This makes blkio's GetStats try to read blkio.io_serviced_recursive
first and fall back to read the stats from blkio.throttle in case it
can't. blkio.io_serviced_recursive seems to be available across all
kernels with CFQ enabled.

This closes #165

Signed-off-by: Johannes 'fish' Ziemke <github@freigeist.org>