core: multiple updates

[abiosoft]

Updates

• Improve disk image downloader
• Convert disk image from qcow2 to raw before starting VM. Should resolve VZ performance issues.
• Use of dedicated and lighter disk image per runtime.
• Introduction of none runtime. colima start --runtime=none to utilise Colima as a virtual machine manager.
• Introduction of incus runtime
• A warning is now displayed for fixed configs that are discarded when changed. VM type, architecture, runtime and volume mount type cannot be changed after the VM is created.