fix block device access

fix block device access

-) The condition was apparently to ignore /dev/ paths while
   that's actually what it was supposed to handle... (other
   paths aren't devices...).
-) Get rid of the blockdevices_list heuristics, it doesn't
   work reliably for all types of devices.
-) Check whether a device is a block device via S_ISBLK on
   the device file.
-) Don't try to follow symlinks as the name we provide in
   the mp config is the name we want in the container.
-) Blacklisted loop devices as they pose a security risk.