tests: iotests: drop some stderr line

tests: iotests: drop some stderr line

In my Out-Of-Band test, "check -qcow2 060" fail with this:

  --- /home/peterx/git/qemu/tests/qemu-iotests/060.out
  +++ /home/peterx/git/qemu/bin/tests/qemu-iotests/060.out.bad
  @@ -427,8 +427,8 @@
  QMP_VERSION
  {"return": {}}
  qcow2: Image is corrupt: L2 table offset 0x2a2a2a00 unaligned (L1
  index: 0); further non-fatal corruption events will be suppressed
  -{"timestamp": {"seconds": TIMESTAMP, "microseconds": TIMESTAMP}, "event": "BLOCK_IMAGE_CORRUPTED", "data": {"device": "", "msg": "L2 table offset 0x2a2a2a0
  0 unaligned (L1 index: 0)", "node-name": "drive", "fatal": false}}
  read failed: Input/output error
  +{"timestamp": {"seconds": TIMESTAMP, "microseconds": TIMESTAMP}, "event": "BLOCK_IMAGE_CORRUPTED", "data": {"device": "", "msg": "L2 table offset 0x2a2a2a0
  0 unaligned (L1 index: 0)", "node-name": "drive", "fatal": false}}
  {"return": ""}
  {"return": {}}
  {"timestamp": {"seconds": TIMESTAMP, "microseconds": TIMESTAMP},
  "event": "SHUTDOWN", "data": {"guest": false}}

The order of the event and the in/out error line is swapped.  I didn't
dig up the reason, but AFAIU what we want to verify is the event rather
than stderr.  Let's drop the stderr line directly for this test.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180620073223.31964-5-peterx@redhat.com>
[Commit message touched up]
Signed-off-by: Markus Armbruster <armbru@redhat.com>