Merge pull request #4381 from qcheng-redhat/update_dd_cmd

Modify dd cmd options to make it more reliable to trigger I/O errors

Author: XueqiangWei

qemu/tests/cfg/blockdev_mirror_src_no_space.cfg

@@ -40,5 +40,5 @@
     post_command = "umount -f ${source_path} && rm -rf ${tmp_image_file} ${source_path}"
     pre_command_timeout = 30
     post_command_timeout = 30
-    write_file_cmd = "dd if=/dev/urandom of=%s/file bs=1M count=200"
+    write_file_cmd = "dd if=/dev/zero of=%s/file bs=1M count=200 oflag=direct"
     error_msg = "No space left on device"