KVM: Bury kvm_{en,dis}able_virtualization() in kvm_main.c once more

KVM: Bury kvm_{en,dis}able_virtualization() in kvm_main.c once more

Now that TDX handles doing VMXON without KVM's involvement, bury the
top-level APIs to enable and disable virtualization back in kvm_main.c.

No functional change intended.

Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Chao Gao <chao.gao@intel.com>
Tested-by: Chao Gao <chao.gao@intel.com>
Tested-by: Sagi Shahar <sagis@google.com>
Link: https://patch.msgid.link/20260214012702.2368778-16-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>