opt-in flag to make .qcow2 images executable

we can think of a new flag to register the .qcow2 files register with `binfmt-misc` and have the system directly execute them, with some sane defaults. So the user will just type

```sh
chown +x myfile.qcow2
./myfile.qcow2
```

and run a virtual machine

The issue names no files, tests, or existing entry points. First clarify the proposed opt-in flag, binfmt-misc registration behavior, and sane defaults, then inspect the command's current option handling. Done means an opted-in executable .qcow2 image launches the virtual machine directly without changing default behavior.

Written by the indexing model from the issue text.