Preset the "Devize size" field in the "Launch Instance" form with the min_disk properties

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.

Currently it is prefilled by the image size only, however a qcow image size is generally always smaller than the min_disk size and launching the instance will fail. the image size should be a fallback if the min_disk property was not set.