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.

nixos/lib/make-disk-image.nix

@@ -188,6 +188,8 @@ let format' = format; in let
       echo "$acc"
     }
 
+    mebibyte=$(( 1024 * 1024 ))
+
     # Approximative percentage of reserved space in an ext4 fs over 512MiB.
     # 0.05208587646484375
     #  × 1000, integer part: 52
@@ -266,10 +268,10 @@ let format' = format; in let
         # Add the GPT at the end
         gptSpace=$(( 512 * 34 * 1 ))
         # And include the bios_grub partition; the ext4 partition starts at 2MB exactly.
-        reservedSpace=$(( gptSpace + 2 * 1024*1024 ))
+        reservedSpace=$(( gptSpace + 2 * mebibyte ))
       '' else if partitionTableType == "legacy" then ''
         # Add the 1MiB aligned reserved space (includes MBR)
-        reservedSpace=$(( 1024*1024 ))
+        reservedSpace=$(( mebibyte ))
       '' else ''
         reservedSpace=0
       ''}
@@ -286,6 +288,14 @@ let format' = format; in let
       requiredFilesystemSpace=$(( diskUsage + fudge ))
 
       diskSize=$(( requiredFilesystemSpace  + additionalSpace ))
+
+      # Round up to the nearest mebibyte.
+      # This ensures whole 512 bytes sector sizes in the disk image
+      # and helps towards aligning partitions optimally.
+      if (( diskSize % mebibyte )); then
+        diskSize=$(( ( diskSize / mebibyte + 1) * mebibyte ))
+      fi
+
       truncate -s "$diskSize" $diskImage
 
       printf "Automatic disk size...\n"