block: Fix bs->request_alignment assertion for bs->sg=1

For sg backends, bs->request_alignment is meaningless and may be 0.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>

block.c

@@ -935,7 +935,7 @@ static int bdrv_open_common(BlockDriverState *bs, BlockDriverState *file,
 
     bdrv_refresh_limits(bs);
     assert(bdrv_opt_mem_align(bs) != 0);
-    assert(bs->request_alignment != 0);
+    assert((bs->request_alignment != 0) || bs->sg);
 
 #ifndef _WIN32
     if (bs->is_temporary) {