block: qcow: Flatten internal and worker modules

Remove the internal and worker submodule layers from the QCOW2
format directory. The former internal files become direct children
of qcow, with internal/mod.rs turning into parser.rs. The worker
backends move up as engine_sync.rs and engine_uring.rs, and
worker/mod.rs, which held only module declarations, is dropped.

The public parser types are now surfaced at the qcow module level,
so external callers use block::formats::qcow instead of reaching
into the internal module.

Assisted-by: Claude:Opus-4.8
Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

cloud-hypervisor/tests/common/utils.rs

@@ -14,7 +14,7 @@ use std::sync::mpsc::Receiver;
 use std::time::{Duration, Instant};
 use std::{cmp, fs, io, panic, thread};
 
-use block::formats::qcow::internal::ImageType as QcowImageType;
+use block::formats::qcow::ImageType as QcowImageType;
 use test_infra::*;
 use vmm_sys_util::tempdir::TempDir;
 #[cfg(not(feature = "mshv"))]