Wrap QcowFile in Arc<Mutex<>> to ensure thread safety when multiple
virtio queues access the same QCOW2 image concurrently.
Previously, each queue received its own QcowSync instance via
new_async_io() that shared the underlying QcowFile through Clone.
However, cloned QcowFile instances share internal mutable state
(L2 cache, reference counts, file seek position) without
synchronization, leading to data corruption under concurrent I/O.
This change serializes all QCOW2 operations through a mutex, which
ensures correctness at the cost of parallelism. A more performant
solution would require separating metadata locking from actual I/O
operations, tracked in #7560.
Related: #7560
Signed-off-by: Anatol Belski <anbelski@linux.microsoft.com>
9bc367a27b