check-raid: fix megaraid_sas detection

precise's 3.11 comes with CONFIG_MEGARAID_SAS=y, rather than "m", so a
megaraid_sas_ioctl exists in /proc/devices (and associated /dev node)
always exist.

The whole check-raid.py logic is completely broken -- megacli knows if
there are no controllers found but the code doesn't allow us to return
and fallback to the next RAID controller (why we would pick just one
anyway?!)

Instead of rewriting it, merely add a stopgap: switch the detection to
check for the presence of a PCI device using sysfs instead.

Change-Id: I5c97473304d1f88174b37ac0b400959849826931