XenServer中的虚拟机QOS限制，仅能够限制虚拟机网络接口的出站速率，不能限制入站速率。
The QoS value limits the rate of transmission from the VM. The QoS setting does not limit the amount of data
the VM can receive. If such a limit is desired, Citrix recommends limiting the rate of incoming packets higher up
in the network (for example, at the switch level).
Citrix XenServer ® 6.2.0 Administrator’s Guide