client.setup_module: Add function import_system_module().

Hi~

Autotest will add its paths to sys.path when testcase is running.
Then we can not import system module if it has same name with
an autotest module.

Such as libvirt module for libvirt api, autotest will import virt.libvirt prior.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>