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

tests/qemu-iotests/060

@@ -33,6 +33,14 @@ _cleanup()
 }
 trap "_cleanup; exit \$status" 0 1 2 3 15
 
+# Sometimes the error line might be dumped before/after an event
+# randomly.  Mask it out for specific test that may trigger this
+# uncertainty for current test for now.
+_filter_io_error()
+{
+    sed '/Input\/output error/d'
+}
+
 # get standard environment, filters and checks
 . ./common.rc
 . ./common.filter
@@ -464,7 +472,7 @@ echo "{'execute': 'qmp_capabilities'}
                         }}" \
             -incoming exec:'cat /dev/null' \
             2>&1 \
-    | _filter_qmp | _filter_qemu_io
+    | _filter_qmp | _filter_qemu_io | _filter_io_error
 
 echo
 # Image should not have been marked corrupt

tests/qemu-iotests/060.out

@@ -428,7 +428,6 @@ 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 0x2a2a2a00 unaligned (L1 index: 0)", "node-name": "drive", "fatal": false}}
-read failed: Input/output error
 {"return": ""}
 {"return": {}}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": "SHUTDOWN", "data": {"guest": false}}