Update the order of importing modules by PEP8.

Wrap the codes to function:
  1. Update function find_disk and rename it.
  2. Update function run_sub_test.
  3. Wrap the part of waiting plugging disks completely to
     function wait_plug_disks.
  4. Wrap the part of creating block devices to function
     create_block_devices.
  5. Wrap the part of getting block devices to function
     get_block_devices.
  6. Wrap the part of hotplug or unplug block devices to
     function plug_block_devices.
  7. Wrap the part of format windows disk to function format_disk_win.
  8. Wrap the part of doing io testing on disk to function
     run_io_test.
  9. Update function get_disk_size.
Signed-off-by: NYongxue Hong <yhong@redhat.com>