virtualbox: Disable KVM enable-at-load

[xtexChooser]

Since Linux kernel 6.12-rc1, KVM has been enabling itself once system boot (torvalds/linux@b4886fa). This leads to VirtualBox failing to start VMs, because VBox conflicts with KVM.

VirtualBox suggests adding kvm.enable_virt_at_load=0 kernel parameter to avoid KVM virtualization being enabled on boot. I suggest to ship this parameter with the virtualbox package, making it ready to use since installed.

[MingcongBai]

I seem to be more inclined to patch the said behaviour in the kernel, there has been some recent discussion about this on the list, too.

Also, since we build the KVM modules into the kernel image (to ease QEMU users), modprobe.d seems difficult.