this gets rid of XS_NO_CCB in sbic(4) by moving to iopools. i dont
have an arch that uses this so i mostly want compile testers, but
if someone actually has the hardware that would be great.
this change is mildly interesting because it demonstrates the
flexibility of iopools at sharing resources between consumers. the
command resources are shared at the kernel level, rather than on a
controller or for a target like you see on most hardware. iopools
will guarantee fair access to the command descriptors across all