Comment 4 for bug 1529449

Since both "drive=" and "serial=" expects an arbitrary string (while the value for "drive=" must be unique since it's the "id=" of a "-drive"), why not use the same string from "drive=" as the value of "serial=" when it's not specified explicitly?

Apparently "-device scsi-hd" has already been doing that (although it does not create the "sn" VPD when a serial is not explicitly specified).