tests/qemu-iotests: Fix qemu-io related output in 026.out.nocache

tests/qemu-iotests: Fix qemu-io related output in 026.out.nocache

qemu-io now prefixes its error and warnings with "qemu-io:".
36b9986b08787019e fixed a lot of iotests output but forget about
026.out.nocache. Fix it too.

Fixes: 99e98d7c9fc1a1639fad ("qemu-io: Use error_[gs]et_progname()")
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190816153015.447957-2-vsementsov@virtuozzo.com
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>