iotests: fix the broken 026.nocache output

This patch fixes longstanding issue with 026 iotest. Unfortunately,
this test contains 2 versions of the correct output, one for cached
writes and one for non-cached ones. People tends to fix only one
version of output of the test and thus noncached version becomes
broken. Unfortunately, it is default in tests/check-block.sh

The following problematic commits were made:
    commit 3b5e14c7
    Author: Max Reitz <mreitz@redhat.com>
    Date:   Tue Dec 2 18:32:51 2014 +0100
    qcow2: Flushing the caches in qcow2_close may fail

    commit a069e2f1
    Author: John Snow <jsnow@redhat.com>
    Date:   Fri Feb 6 16:26:17 2015 -0500
    blkdebug: fix "once" rule

    commit b106ad91
    Author: Kevin Wolf <kwolf@redhat.com>
    Date:   Fri Mar 28 18:06:31 2014 +0100
    qcow2: Don't rely on free_cluster_index in alloc_refcount_block()
Signed-off-by: NPavel Butsykin <pbutsykin@virtuozzo.com>
Signed-off-by: NDenis V. Lunev <den@openvz.org>
CC: Max Reitz <mreitz@redhat.com>
CC: John Snow <jsnow@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>