A variant of the M.2 RAID enablement kit does not manifest with nvme
driver. Address this by allowing 'nvm' subsystype. to allow blank driver.
Also, to be on the safe side, have self.driver always be a string,
so it can be 'falsey' but still work as a string.
Some versions start manifesting nvme devnames with 'c', which
are to be used to interact with multipath to have raw devices
backing a traditional nvme device.
