block: Add bdrv_next_node Similar to bdrv_next, this traverses through graph_bdrv_states. Will be useful to enumerate all the named nodes. Signed-off-by: Fam Zheng <famz@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com> Signed-off-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>