[bugfix] cd_rom.iso_id in resource_vm.go is an optional parameter

[Xiritianming]

No description provided.

[Sczlog]

I am not sure if we should change this to optional, imo a field optional and unconfigured in terraform schema mean it won't be managed by terraform.
If we only change the spec's optional/required field, will make something unexpected happen, for example, in spec we have two cd_rom, appy with iso_id configured, then update iso_id to unconfigured, expected behavior should be change nothing but in current implementation, it will unmount 2 iso.
For now, I prefer to keep it as required and use empty string to indicate cd_rom unmounted.