block: AlignedOperation owns its bounce buffer via Drop

The bounce buffer for an unaligned descriptor was allocated in
execute_async and leaked on error paths, even though, for the sync case
the kernel already had a pointer to the buffer.

Clean this up by moving ownership of the buffer to the AlignedOperation
type. To make it actually safe, stop stashing a guest memory pointer for
the duration of the op. Instead, save the guest address and pass guest
memory back to the complete function.

Signed-off-by: Dylan Reid <dgreid@fb.com>

virtio-devices/src/block.rs

@@ -456,7 +456,9 @@ impl BlockEpollHandler {
 
             let mut request = self.find_inflight_request(desc_index)?;
 
-            request.complete_async().map_err(Error::RequestCompleting)?;
+            request
+                .complete_async(&mem)
+                .map_err(Error::RequestCompleting)?;
 
             let latency = request.start().elapsed().as_micros() as u64;
             let read_ops_last = self.counters.read_ops.load(Ordering::Relaxed);