KVM: selftests: Wrap vcpu_nested_state_get/set functions with x86 guard

struct kvm_nested_state is only available on x86 so far. To be able
to compile the code on other architectures as well, we need to wrap
the related code with #ifdefs.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>