qemu-iotests: place valgrind log file in scratch dir

Do not place the valgrind log file at a predictable path in a
world-writable location. Use the common scratch directory (${TEST_DIR})
instead.
Signed-off-by: NSascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: NBo Tu <tubo@linux.vnet.ibm.com>
Message-id: 1460472980-26319-5-git-send-email-silbe@linux.vnet.ibm.com
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/common.config

@@ -122,7 +122,7 @@ _qemu_img_wrapper()
 
 _qemu_io_wrapper()
 {
-    local VALGRIND_LOGFILE=/tmp/$$.valgrind
+    local VALGRIND_LOGFILE="${TEST_DIR}"/$$.valgrind
     local RETVAL
     (
         if [ "${VALGRIND_QEMU}" == "y" ]; then