Code review comment for ~thibf/uvtool:thibf/uvt-kvm_wait_system_booting

Tried by doing:
```
while uvt-kvm create test release=focal arch=amd64 && uvt-kvm wait bjf-test --insecure --ssh-private-key-file ~/.ssh/id_rsa; do uvt-kvm destroy test; echo cycle done; done
```

Seems reliable. Tried also ssh command, didn't observe any obvious regression.