raw-win32: Fix write request error handling

aio_worker() wrote the return code to the wrong variable.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Tested-by: Guangmu Zhu <guangmuzhu@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

block/raw-win32.c

diff --git a/block/raw-win32.c b/block/raw-win32.c
index 68f2338..b562c94 100644
--- a/block/raw-win32.c
+++ b/block/raw-win32.c
@@ -119,9 +119,9 @@
     case QEMU_AIO_WRITE:
         count = handle_aiocb_rw(aiocb);
         if (count == aiocb->aio_nbytes) {
-            count = 0;
+            ret = 0;
         } else {
-            count = -EINVAL;
+            ret = -EINVAL;
         }
         break;
     case QEMU_AIO_FLUSH: