block: Move QCOW2 format files into formats/qcow/

Move QCOW2 format implementation into a structured directory layout:

  qcow/              -> formats/qcow/internal/  (filenames unchanged)
  qcow_disk.rs       -> formats/qcow/mod.rs               (QcowDisk)
  qcow_sync.rs       -> formats/qcow/worker/sync.rs       (QcowSync)
  qcow_async.rs      -> formats/qcow/worker/async_uring.rs (QcowAsync)
  qcow_common.rs     -> formats/qcow/common.rs

All internal cross references continue to resolve through
re-exports in lib.rs: formats::qcow::internal as qcow,
formats::qcow as qcow_disk, and
formats::qcow::common as qcow_common.

Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>

block/src/formats/mod.rs

@@ -7,6 +7,7 @@
 //! Each format lives in its own submodule with a `DiskFile` wrapper,
 //! format specific internals, and sync/async I/O workers.
 
+pub mod qcow;
 pub mod raw;
 pub mod vhd;
 pub mod vhdx;