make-disk-image: Round image size to the next mebibyte
make-disk-image: Round image size to the next mebibyte

This ensures the following gptfdisk warning won't happen:

```
Warning: File size is not a multiple of 512 bytes! Misbehavior is likely!
```

Additionally, helps towards aligning the partition to be more optimal
for the underlying storage.

It is actually impossible to align for the actual underlying storage
optimally because we don't know what the block device will be!

But aligning on 1MiB should help.

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