Extract kernel and cmdline from guest disk

As firecracker itself requires the kernel to be provided from the host, firectl relays this requirement. This is okay for environments where either one controls both the hypervisor and the guest, or where it is not desired that the VM itself may provide a kernel[1].

I would however like to be able to pass only a root drive to firectl and have it extract the kernel and/or cmdline from a well-defined path (e. g. `/boot/vmlinux` and `/boot/cmdline`). For example, I stumbled upon this issue when I created a VM with the kernel configured as per [the sample config](https://github.com/firecracker-microvm/firecracker/blob/1a325f4/resources/microvm-kernel-config), and it wouldn’t mount my XFS root.

I am going to implement this anyway, but I’m glad to submit a patch if you’re not opposed to this feature per se.

[1]: I’m not sure whether kexec would work from inside, though …