It's checking that the VDI requested exists on the SR that we've told XSM we want to migrate to.

dest_sr_ref is provided to the migrate_send function and must be the remote ref (personally I think this really should have been the UUID to remove uncertainties) - dest_vdi_sr_ref is therefore the SR which the VDI already exists on, and the check is that the target VDI is on the correct SR on the remote end.