KubeVirt KKP KubevirtNodeSpec add `omitempty` to optional fields

[hdurand0710]

We would like to change the KubevirtNodeSpec struct to :

• add omitempty to optional fields.

We would like to do this for all existing fields. Note that it's already the case for other Providers, that would be an alignment of KubeVirt provider with others.

This requires to check with the UI team and coordinate with them to see if they need to do some changes on their side too.

Acceptance Criteria:

• check is done with the UI team and a dahsboard User Story is created if needed.
• synchronisation with UI team:
  • if changes are required on their side (accept fields that are not sent) -> wait for their change to be released.
  • move to omitempty when UI is released.
• optional fields are marshalled with omitempty