[bug report]scsi: drive letter drift problem

[Wenchao Hao <haowenchao@xxxxxxxxxx>]

We deploy two SCSI disk in one SCSI host(virtio-scsi bus) for one 
machine, whose ids are [0:0:0:0] and [0:0:1:0].

Mostly, the device letter are assigned as following after reboot:
[0:0:0:0] --> sda
[0:0:1:0] --> sdb

While in rare cases, the device letter shown as following:
[0:0:0:0] --> sdb
[0:0:1:0] --> sda

Could we guarantee "sda" is assigned to [0:0:0:0] and "sdb" is assigned 
to [0:0:1:0] or not?
If we can, then how?