qemu.tests: Add heavy disk write load test for windows guests

Add heavy disk write load test for windows guests. Using Crystal
Disk Mark [1] to generate the write load inside the guest. And set the
default cache to writeback in this test.

changes from v1:
  - Record steps in INFO logs
  - Close the session after test
  - Add command empty check

[1] Crystal Disk Mark is a BSD licensed program
    http://crystalmark.info/software/CrystalDiskMark/manual-en/License.html

    It can be downloaded here:
    http://crystalmark.info/software/CrystalDiskMark/index-e.html

    The file will be added to the winutils.iso

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Acked-by: Feng Yang <fyang@redhat.com>