Save the contents of the FC fifos to files

[luxas]

Firecracker emits one fifo for metrics and one for VMM logs. Both of these are by nature very ephemeral. ignite-spawn should read the fifos periodically (or better, stream) and write the contents to persistent log files in /var/lib/firecracker/vm/$id/{vmm,metrics}.log

[luxas]

Hi @patrobinson would you like to help with this issue?
Basically, it's about opening the two FIFO files Firecracker emits, reading them using https://github.com/containerd/fifo; and streaming the read content to a persistent file, e.g. a file with the same name, but ending with .log and not .fifo.

What do you think? 😄