Use the min_disk property for volume size

In the "Launch Instance" form/wizard, when selecting "Boot from image (creates
a new volume)" the "Device size" field should be prefilled with the highest
value among the "min_disk" properties of the image, and the image's size,
instead of the image size only.

This is because the image size is generaly a qcow compressed image and in
such case almost always far smaller than the min_disk porperty.

Change-Id: Ia18fd8f7550863177167ce2b0e727d064889b61c
Implements: bp prefill-create-a-new-volume-device-size-min-requirements
Merge "Use the min_disk property for volume size"