selftests: KVM: Remove unused x86_register enum

x86_register enum is not used, let's remove it.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Suggested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>