KVM: selftests: Convert triple_fault_event_test away from VCPU_ID

Convert triple_fault_event_test to use vm_create_with_one_vcpu() and pull
the vCPU's ID from 'struct kvm_vcpu'.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>