Sandboxes start from your larger images with no disk to name

A sandbox created from an image of yours larger than the default 4 GiB disk, with no diskMiB, was refused disk_too_small. An image of up to 8 GiB now gets an 8 GiB disk, within the 10 GiB a running sandbox's disk includes, and starts from the image's started copy of that size. A larger image is still refused, now naming the diskMiB to ask for; a diskMiB you name is kept. See images.