block: Remove BlockDriverState.blk

This patch removes the remaining users of bs->blk, which will allow us
to have multiple BBs on top of a single BDS. In the meantime, all checks
that are currently in place to prevent the user from creating such
setups can be switched to bdrv_has_blk() instead of accessing BDS.blk.

Future patches can allow them and e.g. enable users to mirror to a block
device that already has a BlockBackend on it.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>

block.c

@@ -2226,14 +2226,6 @@ static void change_parent_backing_link(BlockDriverState *from,
 {
     BdrvChild *c, *next;
 
-    if (from->blk) {
-        /* FIXME We bypass blk_set_bs(), so we need to make these updates
-         * manually. The root problem is not in this change function, but the
-         * existence of BlockDriverState.blk. */
-        to->blk = from->blk;
-        from->blk = NULL;
-    }
-
     QLIST_FOREACH_SAFE(c, &from->parents, next_parent, next) {
         assert(c->role != &child_backing);
         c->bs = to;
@@ -2875,7 +2867,7 @@ const char *bdrv_get_node_name(const BlockDriverState *bs)
     return bs->node_name;
 }
 
-static const char *bdrv_get_parent_name(const BlockDriverState *bs)
+const char *bdrv_get_parent_name(const BlockDriverState *bs)
 {
     BdrvChild *c;
     const char *name;