blktrace: avoid device duplication

blktrace: avoid device duplication

Fixup for bz 501457.

Problem:
If the device list file contains the same device
as supplied on the command line, blktrace stops
immediately and further I/O tracing is impossible.

Bug: device duplication in the devpaths ends with
programm termination (BLKTRACESETUP ioctl returns
error) while resources (open files in debugfs) are
not released.

Solution:
Make sure devices are not duplicated in devpaths
pool.

Signed-off-by: Edward Shishkin <edward@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>