selftests: kvm: report failed stage when exit reason is unexpected

When we get a report like

==== Test Assertion Failure ====
x86_64/state_test.c:157: run->exit_reason == KVM_EXIT_IO
pid=955 tid=955 - Success
1 0x0000000000401350: main at state_test.c:154
2 0x00007fc31c9e9412: ?? ??:0
3 0x000000000040159d: _start at ??:?
Unexpected exit reason: 8 (SHUTDOWN),

it is not obvious which particular stage failed. Add the info.

Signed-off-by: Vitaly Kuznetsov <[e-mail]>
Signed-off-by: Radim Krčmář <[e-mail]>

tools/testing/selftests/kvm/x86_64/evmcs_test.c

···
 	for (stage = 1;; stage++) {
 		_vcpu_run(vm, VCPU_ID);
 		TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
-			    "Unexpected exit reason: %u (%s),\n",
-			    run->exit_reason,
+			    "Stage %d: unexpected exit reason: %u (%s),\n",
+			    stage, run->exit_reason,
 			    exit_reason_str(run->exit_reason));
 
 		memset(&regs1, 0, sizeof(regs1));

tools/testing/selftests/kvm/x86_64/state_test.c

···
 	for (stage = 1;; stage++) {
 		_vcpu_run(vm, VCPU_ID);
 		TEST_ASSERT(run->exit_reason == KVM_EXIT_IO,
-			    "Unexpected exit reason: %u (%s),\n",
-			    run->exit_reason,
+			    "Stage %d: unexpected exit reason: %u (%s),\n",
+			    stage, run->exit_reason,
 			    exit_reason_str(run->exit_reason));
 
 		memset(&regs1, 0, sizeof(regs1));