qemu.tests: Make vhost_with_cgroup works for multiqueue

Make it can work well with multiqueue and also add some debug message
when test case is failed.

Signed-off-by: Yiqiao Pu <ypu@redhat.com>
Merge pull request #39 from ypu/vhost_with_cgroup

qemu.tests: Make vhost_with_cgroup works for multiqueue