kernel: give any additional arguments to the kernel

This seems more useful than for KVM. Well, both are useful, we could
give arguments starting with a dash to KVM and others to the kernel but
there is the problem of additional arguments for each KVM option...
kernel: ability to give arguments to kernel and to KVM

Arguments for kernel are separated from arguments for KVM by `--`.
kernel: first argument is a kernel only if it is a file

Otherwise, we consider that this is additional arguments. Should work
most of the time.