make libkrun the default when creating a podman machine

[benoitf]

Is your enhancement related to a problem? Please describe

switch from applehv to libkrun as default

I don't see any regression for now and it will still be a user choice

Describe the solution you'd like

set the containers.conf default provider as well if it's the first time or if there is no other machine existing So it'll be in sync with what the cli is seeing

Describe alternatives you've considered

keep it like this but then people wanting to try tu run AI models need to redo machines afterwards etc.

Additional context

No response