Added utility in disk library help to Set io scheduler to a device

Added utility in disk library help to Set io scheduler to a device

Signed-off-by: Praveen K Pandey <praveen@linux.vnet.ibm.com>

avocado/utils/disk.py

@@ -474,4 +474,17 @@ def get_io_scheduler(device_name):
 
 def __sched_path(device_name):
 
-    return "/sys/block/%s/queue/scheduler" % device_name
+    return "/sys/block/%s/queue/scheduler" % device_name
+
+
+def set_io_scheduler(device_name, name):
+    """
+    Set io scheduler to a device
+    :param device_name:  Device  name example like sda , hda
+    :param name: io scheduler name
+    """
+    if name not in get_io_scheduler_list(device_name):
+        raise DiskError("No such IO scheduler: %s" % name)
+
+    with open(__sched_path(device_name), "w") as fp:
+        fp.write(name)