iotests: Fix test 178

A recent change to qemu-img changed expected error message output, but
178 takes long enough to execute that it does not get run by 'make
check' or './check -g quick'.

Fixes: 43d589b074
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20200521192137.1120211-2-eblake@redhat.com>
2 files changed