Add testcases for cloudimg-rootfs feature using nullboot and generic/kvm kernel

[xnox]

cloudimg-rootfs is currently not tested in core-initrd project. But it should be. Ideally by self-building kernel.efi for generic or kvm kernel, and using nullboot to deploy bootloader and reboot.

[xnox]

it seems very hard to make this work with kvm kernel

so this needs to be with generic kernel.