It is reported that

command with only cdrom in virtual machine doesn't work without argument, but happily accepts empty string as argument.

vws cdrom --eject tstmchn

I

It sueems that more than one cdrom is quite rare.

Co may be use --eject without argument and requiire some other option to specify device in these rare cases

vws cdrom tstmachine --eject --device ide-cd0

It found out that syntax is designed as descripbed above, but implementation is bad. IT stores special value None into file field of Namespace.

Better to use separate field to indicate eject.