block_attach: check file is aligned

[reynir]

Otherwise if the file is not a multiple of block_size (512) we have to make up bytes for the missing part.

[reynir]

After testing it turns out that unlike mirage-block-unix solo5 doesn't conjure up magic zero bytes but instead "truncates" the block device. That is, a 1000 byte file will be seen as a block device consisting of a single 512 byte sector(!)

[dinosaure]

Thanks!