In multiRU we will need to be able to check multiple devices and DMA channels,
not only /dev/sdr0@0 that was implicitly used until now.

-> As a preparatory step adjust the SR to explicitly specify which SDR resource is being verified.

This patch is necessary because in nexedi/slapos.toolbox!125
we adjust check_sdr_busy plugin to require SDR device/DMA channel to be
explicitly specified.

/cc @lu.xu, @tomo, @xavier_thompson, @Daetalus
/reviewed-by @jhuge
/reviewed-on nexedi/slapos!1458