Review/fix validation of image without checksum

This serie fix the issue #50.

I added some log_mesg() to help to diagnose what we get in the image.

However, the checksum of the restored ext4 file system is not identical to the cloned one, when it is run on a 32 bits host. The original file system contains some bits that are not taken into account in bitmap. The log contains some messages likes: "extfsclone.c: BLOCK_UNINIT for group 2". This is also the case on the release branch (ie it was not introduced by the "format 0002").