tests: Update clean up session to serial session

As this guest performance will get worse when the disk is filled up,
and may influence the normal session. Use serial session instead.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
tests: add fillup_disk test steps info logging.info

Record test steps into logging.info to make steps looks more clearly;
And fix some coding style issue;

Signed-off-by: Xu Tian <xutian@redhat.com>
Acked-by: Suqin Huang <shuang@redhat.com>