generic.tests.autotest_control: Support args parameters when running client tests in guest.

run_autotest() support this feature by update the control file used in
guest.
We can add args by set control_args.
control_args = "-rw=randread -bs=8k -size 1G -numjobs=8 -runtime=600"

Signed-off-by: Feng Yang <fyang@redhat.com>
Merge pull request #163 from FengYang/autotest_control

generic.tests.autotest_control: Support args parameters when running cli...