base: warn on megacli unknown controllers

It turns out that the megacli version that we currently run doesn't
recognize the controller on some of the newer boxes. When that happens,
the output is empty and instead of OK'ing it, we should issue a WARNING.

getLinuxUtility() makes sure that the function only gets called if
megaraid_sas_ioctl is present on /proc/devices, so a warning will
basically mean the kernel & megacli disagreeing whether a controller is
found, something that sounds sensible.

Change-Id: I0dadceba733eab025637f413e0132658ca50f235