block: Fix multiwrite memory leak in error case

Previously multiwrite_user_cb was never called if a request in the multiwrite
batch failed right away because it did set mcb->error immediately. Make it look
more like a normal callback to fix this.
Reported-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

block.c

@@ -1884,7 +1884,8 @@ int bdrv_aio_multiwrite(BlockDriverState *bs, BlockRequest *reqs, int num_reqs)
                 reqs[i].error = -EIO;
                 goto fail;
             } else {
-                mcb->error = -EIO;
+                mcb->num_requests++;
+                multiwrite_cb(mcb, -EIO);
                 break;
             }
         } else {