Instance creation: Disk size is not shown for existing disks to be attached

[askfongjojo]

The disk size information should be available for existing disks and displayed in the disk section shown above.

[zephraph]

After some initial investigation this is because we're including info from the attach parameters directly to this table. That means we technically don't have the attached image size at this point and it'll need to be fetched separately.

Seeing this made me realize we don't accept ID when attaching the disk so I'll need to go through and add that to the API. The other realization here is that we actually have already looked up the disk (via the list required to retrieve it from the dropdown). I'll see about passing that info through to the component to fill out the rest of the data.

[charliepark]

This will be fixed when #2841 is merged to main