block: assert that bs->request_alignment is a power of 2

at least bdrv_co_preadv/pwritev expect this.
Signed-off-by: NPeter Lieven <pl@kamp.de>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -1018,7 +1018,7 @@ static int bdrv_open_common(BlockDriverState *bs, BdrvChild *file,
 
     assert(bdrv_opt_mem_align(bs) != 0);
     assert(bdrv_min_mem_align(bs) != 0);
-    assert((bs->request_alignment != 0) || bdrv_is_sg(bs));
+    assert(is_power_of_2(bs->request_alignment) || bdrv_is_sg(bs));
 
     qemu_opts_del(opts);
     return 0;