block: qcow: Add pread_alloc and decompress_cluster helpers

Add two reusable helpers for the compressed cluster read path:

- pread_alloc(fd, offset, len) allocates a buffer and fills it with
  pread_exact, returning the owned Vec.
- decompress_cluster(compressed, cluster_size, decoder) allocates the
  output buffer, decodes via the Decoder trait, and validates that the
  decoder produced exactly cluster_size bytes.

These will be used by QcowSync, QcowAsync, Qcow2Backing, and the
legacy QcowFile.

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

block/src/qcow/mod.rs

@@ -5,7 +5,7 @@
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
 pub(crate) mod backing;
-mod decoder;
+pub(crate) mod decoder;
 mod header;
 pub(crate) mod metadata;
 pub(crate) mod qcow_raw_file;