Fix diskimage-builder image size
Fix diskimage-builder image size

This patch fixes the calculation of the resultant image size
when building an image with diskimage-builder on ext4 a
filesystem.

Prior to this, using the '--image-size 2' (2GB) setting would
generate an image that would not boot under a 2GB nova flavor.

Change-Id: I7a753bdef84c6300ccea73ae4a92bf330dcd77cb
Closes-Bug: #1513622

switch-ch/diskimage-builder,openstack/diskimage-builder,takahashinobuyuki/diskimage-builder,openstack/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,takahashinobuyuki/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,zioproto/diskimage-builder,switch-ch/diskimage-builder,zioproto/diskimage-builder