qemu-iotests: check: don't place files with predictable names in /tmp

Placing files with predictable or even hard-coded names in /tmp is a
security risk and can prevent or disturb operation on a multi-user
machine. Place them inside the "scratch" directory instead, as we
already do for most other test-related files.

Signed-off-by: Sascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: Bo Tu <tubo@linux.vnet.ibm.com>
Message-id: 1459848109-29756-2-git-send-email-silbe@linux.vnet.ibm.com
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
