block: avoid useless checks on acb->bh

Coverity is confused by this "if" and reports leaks on acb->bh.
The bottom half is always deleted before releasing the AIOCB,
in either bdrv_aio_cancel_em or bdrv_aio_bh_cb.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -3077,9 +3077,7 @@ static BlockDriverAIOCB *bdrv_aio_rw_vector(BlockDriverState *bs,
     acb->is_write = is_write;
     acb->qiov = qiov;
     acb->bounce = qemu_blockalign(bs, qiov->size);
-
-    if (!acb->bh)
-        acb->bh = qemu_bh_new(bdrv_aio_bh_cb, acb);
+    acb->bh = qemu_bh_new(bdrv_aio_bh_cb, acb);
 
     if (is_write) {
         qemu_iovec_to_buffer(acb->qiov, acb->bounce);