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 <vkuznets@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>

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

@@ -113,8 +113,8 @@ int main(int argc, char *argv[])
 	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

@@ -152,8 +152,8 @@ int main(int argc, char *argv[])
 	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));