frontend: refactor VmListModel#onSave method

Extract code from VmListModel#onSave to two separate methods:
- saveNewVm which adds VM as new entity
- updateExistingVm which updates existing VM entity

It makes the code more maintainable, as the onSave method was too long
and had too many responsibilities.

Change-Id: I90a0164146fd17eb2dced71699bbf3d8ac489004
Signed-off-by: Arik Hadas <ahadas@redhat.com>