storage: Add reproducer for https://github.com/Code-Hex/vz/issues/201

At the moment, when using Code-Hex/vz like this:
```
f, err := os.OpenFile(devPath, open_flags, 0)
if err != nil {
	return nil, fmt.Errorf("error opening file: %v", err)
}

attachment, err := vz.NewDiskBlockDeviceStorageDeviceAttachment(f, conf.ReadOnly, vz.DiskSynchronizationModeFull)
if err != nil {
	_ = f.Close()
	return nil, fmt.Errorf("error creating disk attachment: %v", err)
}
```

the developer has to know that `f` must be kept alive at least as long as the disk attachment is in use. If not, `f` will be garbage collected, its file descriptor will be closed, and the attachment becomes invalid. In this situation, the virtualization framework would return an error.

This commit simply adds a test case exhibiting this problem. This will
be improved in the next commits.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>