block: qcow: Extract positional I/O helpers into a common module

These position independent I/O helpers use pread64/pwrite64 to avoid
races on the shared file position when multiple queues operate on
duplicated file descriptors. Extracting them prepares for reuse by
the upcoming qcow_async backend.

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

block/src/lib.rs

@@ -18,6 +18,7 @@ pub mod fixed_vhd;
 pub mod fixed_vhd_async;
 pub mod fixed_vhd_sync;
 pub mod qcow;
+pub(crate) mod qcow_common;
 pub mod qcow_sync;
 #[cfg(feature = "io_uring")]
 /// Async primitives based on `io-uring`