Use megabyte granularity for image extra space

I want to use the new --image-extra-size flag[1] but my use-case
calls for megabyte granularity of this value. Rather than adding
60% to an 800MB image, maybe I only want to add 100 or 200MB, etc.

[1] https://review.opendev.org/#/c/655127/

Change-Id: I8fb9685d60ebb1260d5efcf03c5c23c561c24384

diskimage_builder/lib/disk-image-create

@@ -381,8 +381,8 @@ else
     sudo du -a -c -x ${TMP_BUILD_DIR}/built > ${du_output}
     # the last line is the total size from "-c".
     if [ -n "$DIB_IMAGE_EXTRA_SIZE" ]; then
-        # add DIB_IMAGE_EXTRA_SIZE to create a bigger image as requested
-        du_extra_size=$(echo "$DIB_IMAGE_EXTRA_SIZE" | awk '{printf("%d\n",$1 * 1024 *1024)}')
+        # add DIB_IMAGE_EXTRA_SIZE megabytes to create a bigger image as requested
+        du_extra_size=$(echo "$DIB_IMAGE_EXTRA_SIZE" | awk '{printf("%d\n",$1 * 1024)}')
         du_size_tmp=$(tail -n1 ${du_output} | cut -f1)
         du_size=$(echo "$du_size_tmp $du_extra_size" | awk '{print int($1 + $2)}')
     else

doc/source/user_guide/building_an_image.rst

@@ -627,7 +627,7 @@ following arguments:
 
 ``--image-extra-size``
    Extra space to add when automatically calculating image size, in
-   gigabytes.  This overrides the default 60% scale up as described
+   megabytes.  This overrides the default 60% scale up as described
    above for ``--image-size``.  Can also set ``DIB_IMAGE_EXTRA_SIZE``.
 
 The special node named ``mkfs_root`` is affected by the following;

releasenotes/notes/image-size-padding-mb-574104d40fdd2345.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    The ``--image-extra-size`` argument has changed from gigabytes to
+    megabytes to make it more practical for use on smaller images.