qemu.tests: add vhost cgroup test

This patch make case "[virtual NIC device] vhost with cgroup" auto

ChangLog:
    change from V1:
        1.replace range with xrange
        2.delete unused variant session
        3.add more error context
    change from V2:
        1.after guest was booted then assign the vm to cgroup
        2.check when no vhost-pid exist
    change form V3:
        1.modify funticon assign_vm_into_cgroup

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>
Acked-by: Suqin Huang <shuang@redhat.com>