On success aio_discard returns 0.

Signed-off-by: Mykola Golub <mgolub@mirantis.com>

- parent_wait() returns child exit status so test for "!= 0";
- do_map() is expected to return a negative value on failure.

Signed-off-by: Mykola Golub <mgolub@mirantis.com>

- cleanup on a test failure;
- minimize interference with other processes (tests) that are
  run concurrently;
- use xmlstarlet when parsing rbd output;
- add exit status test.

Signed-off-by: Mykola Golub <mgolub@mirantis.com>