block: move supports_backing check to bdrv_set_file_or_backing_noperm()

Move supports_backing check of bdrv_reopen_parse_backing to called
(through bdrv_set_backing_noperm()) bdrv_set_file_or_backing_noperm()
function. The check applies to general case, so it's appropriate for
bdrv_set_file_or_backing_noperm().

We have to declare backing support for two test drivers, otherwise new
check fails.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210610120537.196183-7-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/unit/test-bdrv-graph-mod.c

@@ -41,6 +41,7 @@ static void no_perm_default_perms(BlockDriverState *bs, BdrvChild *c,
 
 static BlockDriver bdrv_no_perm = {
     .format_name = "no-perm",
+    .supports_backing = true,
     .bdrv_child_perm = no_perm_default_perms,
 };