KVM: selftests: Assert that __vm_get_stat() actually finds a stat

Fail the test if it attempts to read a stat that doesn't exist, e.g. due
to a typo (hooray, strings), or because the test tried to get a stat for
the wrong scope.  As is, there's no indiciation of failure and @DaTa is
left untouched, e.g. holds '0' or random stack data in most cases.

Fixes: 8448ec5 ("KVM: selftests: Add NX huge pages test")
Link: https://lore.kernel.org/r/20250111005049.1247555-4-seanjc@google.com
[sean: fixup spelling mistake, courtesy of Colin Ian King]
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/lib/kvm_util.c

@@ -2238,9 +2238,10 @@ void __vm_get_stat(struct kvm_vm *vm, const char *stat_name, uint64_t *data,
 
 		read_stat_data(vm->stats_fd, &vm->stats_header, desc,
 			       data, max_elements);
-
-		break;
+		return;
 	}
+
+	TEST_FAIL("Unable to find stat '%s'", stat_name);
 }
 
 __weak void kvm_arch_vm_post_create(struct kvm_vm *vm)