This implementation works fine for increasing the disk size.
However, it returns an error when shrinking, as disk shrinking is not supported.

Needs rebase

Summary of Latest Review:

1. DiskDir is dedicated to additional disks.
   • I've updated the implementation by creating a new ResizeDiffDisk function, which directly resizes the diffdisk.
2. Calling inst.inspect() twice:
   • The editAction function returns instance.Start which doesn’t update the validated configuration in the store or the instance yet.
   • The second call to inst.inspect() is to apply the new configuration.
   • The first call to inst.inspect() checks whether the instance exists and is running.

Note that #3513 moved the qemu-img functions to pkg/qemu/imgutil.

I pushed the same code as before cleaning up the commits, but the test workflow is still failing.

Thanks a lot for reviewing!