cloud-hypervisor: cannot override the `--serial` and `--console` args.

[Ramblurr]

Issue

The cloud-hypervisor runner unconditionally sets --serial and --console.

https://github.com/astro/microvm.nix/blob/f30ae9b9e740116d044935c9c0c68bd1c2ce8432/lib/runners/cloud-hypervisor.nix#L142-L143

Motivation

...but there are other values that could be useful (at least for myself)

      --console <console>
          Control (virtio) console: "off|null|pty|tty|file=</path/to/a/file>,iommu=on|off" [default: tty]
      --serial <serial>
          Control serial port: off|null|pty|tty|file=</path/to/a/file>|socket=</path/to/a/file> [default: null]

Also see the slow boot problem caused by the default --serial tty in #366

Current behavior

If you try to set --serial for example, you get the error:

$ nix run .#packages.x86_64-linux.cloud-hypervisor-example --show-trace
warning: Git tree '/home/ramblurr/src/microvm.nix' is dirty
error: the argument '--serial <serial>' cannot be used multiple times

Usage: microvm@cloud-hypervisor-microvm [OPTIONS]

For more information, try '--help'.

Possible solutions

I understand the goal not to have an additional nixos module option for every single CLI flag for the VMMs, that would just get out of hand.

I think a good solution is to detect if the user passed a flag in the extraArgs list, and if so, do not add the default that microvm otherwise would.

This is easy to implement now using the extractOptValues function I implemented over in #336

If you approve this, then I can prepare a PR that does exactly that.

A PR for this is over in #377 so you can see exactly what this looks like.