Update guest kernel and rootfs

[ustiugov]

Need to update the kernel, following the recipe from the Firecracker team.

We should update the rootfs, following the firecracker-container recipe, as Plamen suggested.

[plamenmpetrov]

I believe that the above-mentioned recipe will not be enough, as the agent needs to be part of the rootfs and it should also start when the MicroVM boots. The firecracker-containerd recipe might be helpful.

[ustiugov]

@plamenmpetrov that's a good point, updated.