KVM: selftests: Add a test to get/set triple fault event

Add a selftest for triple fault event:
  - launch the L2 and exit to userspace via I/O.
  - using KVM_SET_VCPU_EVENTS to pend a triple fault event.
  - with the immediate_exit, check the triple fault is pending.
  - run for real with pending triple fault and L1 can see the triple
    fault.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Chenyi Qiang <chenyi.qiang@intel.com>
Message-Id: <20220524135624.22988-3-chenyi.qiang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>