nuageinit: Silence an fstyp(8) warning

Silence a warning emitted by fread(3) in fstyp(8)'s read_buf(), when
detecting the file system type of the cloud-init device:

% fstyp /dev/iso9660/cidata
fstyp: fread: Invalid argument
cd9660

Also rephrase slightly a comment while here.

Signed-off-by: Jose Luis Duran <jlduran@gmail.com>