Taking in a filehandle gives the user quite a bit more freedom on how to
handle boot log output. They can set up a kqueue watch on it and
redirect output somewhere else etc etc. The implementation for this has
us take in a new BootLog type that has two options:
1. .file, which is analogous to what we had prior. Just provide a URL
and a true by default append field.
2. .fileHandle which is the new addition. Can pass any fd that is
writable, and the VMM should write serial console output to it.
Danny Canter
4c761d50c6