Replace unwrap panics with proper error propagation

[kirgene]

I'm using qcow2-rs as a library via rublk to expose qcow2 images as ublk block devices. When rublk hits a corrupt or malformed image, the unwrap() in the IO backend's new() panics and kills the daemon. Since the kernel-side ublk device is already registered at that point, the orphaned device causes unkillable D-state processes — only a reboot fixes it.

This patch replaces unwrap() with proper error propagation across all IO backends and the setup macros, so callers get a clean error instead of a process crash.

Also fixes rqcow2 check — it was printing "non-allocated cluster" warnings but still returning exit 0, which made it useless for scripted pre-validation. Now it returns exit 1 on integrity errors.

Changes:

• Qcow2IoSync::new(), Qcow2IoTokio::new(), Qcow2IoUring::new() return Qcow2Result instead of panicking on file open
• qcow2_setup_dev_fn_sync! and qcow2_setup_dev_fn! macros propagate the error with ?
• check_cluster() returns Err when a cluster is non-allocated (was Ok(()))
• main() prints the error and exits 1 instead of panicking

Tested against a mix of valid and corrupt qcow2 images.

[ming1]

Hello,

The build is failed:

error[E0599]: no method named `read_to` found for enum `Result` in the current scope
    --> src/dev.rs:2957:24
     |
2957 |             let _ = io.read_to(0, &mut buf).await;
     |                        ^^^^^^^ method not found in `Result<Qcow2IoTokio, Qcow2Error>`
     |
note: the method `read_to` exists on the type `Qcow2IoTokio`
    --> src/ops.rs:19:5
     |
19   |     async fn read_to(&self, offset: u64, buf: &mut [u8]) -> Qcow2Result<usize>;
     |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     = help: items from traits can only be used if the trait is implemented and in scope
note: `ops::Qcow2IoOps` defines an item `read_to`, perhaps you need to implement it
    --> src/ops.rs:18:1
     |
18   | pub trait Qcow2IoOps {
     | ^^^^^^^^^^^^^^^^^^^^
help: consider using `Result::expect` to unwrap the `Qcow2IoTokio` value, panicking if the value is a `Result::Err`
     |
2957 |             let _ = io.expect("REASON").read_to(0, &mut buf).await;
     |                       +++++++++++++++++

For more information about this error, try `rustc --explain E0599`.
error: could not compile `qcow2-rs` (lib test) due to 1 previous error
warning: build failed, waiting for other jobs to finish...

Thanks,

[kirgene]

Fixed, added unwrap() to the test.