block: Avoid printing NULL string in error messages

Even for nodes that have a BlockBackend attached, bdrv_get_parent_name()
can return NULL if the BB is anonymous (e.g. it belongs to a block job
or a device that was created with a drive=<node-name> option).

Remove the information from the error message. The user probably knows
already why the node is still in use.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>