Add service module in virttest.(Base on service module in client.shared)

This service module is based on autotest.client.shared.service.
virt-service module extend the client-service to manager the
service on remote host (guest).

TODO: Not return CmdResult object immediatly.
Make the result of command in COMMANDS more useful.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>