virttest.test_setup: Add KSM setup class

This class will setup and clean KSM config in different hosts.
You can use setup_ksm to configure it. There are several params
that can be used for ksm configure:

ksm_pages_to_scan: pages ksmd will scan in one loop
ksm_sleep_ms: million secs ksmd will sleep between two scan
ksm_run: start ksmd or not
ksm_module: if ksm is not compiled into kerenl need this parameter
            tell scripts which module should it load
disable_ksmtuned: if we need idsable ksmtuned during our test

This class also can be used to disable KSM in some special case with
set setup_ksm to yes and ksm_run to 0 or no.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>