Honor $DIB_IMAGE_SIZE.

The recent change to use loopdev instead of nbd stopped
honoring the DIB_IMAGE_SIZE setting.

This change adds it back, by resizing the image to
DIB_IMAGE_SIZE GB, if specified. If unspecified, it resizes
the image to (actual usage + 20%), as it did before this change.

Change-Id: I69afd9584e644ddacc948619100f153d3d8713a4
Merge "Honor $DIB_IMAGE_SIZE."