config: enable KVM host support on x86_64

Enable CONFIG_KVM, CONFIG_KVM_INTEL, and CONFIG_KVM_AMD as built-in
so the guest kernel can act as a KVM host. This is needed for nested
virtualization: when libkrun exposes VMX/SVM CPUID bits to the guest
(via krun_set_nested_virt), the guest kernel needs KVM support to
create /dev/kvm and run VMs.

Both Intel and AMD variants are enabled so the same kernel binary
works on either vendor.

Assisted-by: <anthropic/claude-opus-4.6>
Signed-off-by: Dusty Mabe <dusty@dustymabe.com>