block: Don't silently truncate node names

If the user passes a too long node name string, we silently truncate it
to fit into BlockDriverState.node_name, i.e. to 31 characters. Apart
from surprising the user when the node has a different name than
requested, this also bypasses the check for duplicate names, so that the
same name can be assigned to multiple nodes.

Fix this by just making too long node names an error.

Reported-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>