KVM: Unexport kvm_vcpu_cache and kvm_vcpu_{un}init()

Unexport kvm_vcpu_cache and kvm_vcpu_{un}init() and make them static
now that they are referenced only in kvm_main.c.

Acked-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>