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

Add heavy disk write load test for windows guests. Using Crystal
Disk Mark 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

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Acked-by: Feng Yang <fyang@redhat.com>