KVM: selftests: Fix mostly theoretical leak of VM's binary stats FD

When allocating and freeing a VM's cached binary stats info, check for a
NULL descriptor, not a '0' file descriptor, as '0' is a legal FD.  E.g. in
the unlikely scenario the kernel installs the stats FD at entry '0',
selftests would reallocate on the next __vm_get_stat() and/or fail to free
the stats in kvm_vm_free().

Fixes: 83f6e10 ("KVM: selftests: Cache binary stats metadata for duration of test")
Link: https://lore.kernel.org/r/20250111005049.1247555-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

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

@@ -749,7 +749,7 @@ void kvm_vm_free(struct kvm_vm *vmp)
 		return;
 
 	/* Free cached stats metadata and close FD */
-	if (vmp->stats_fd) {
+	if (vmp->stats_desc) {
 		free(vmp->stats_desc);
 		close(vmp->stats_fd);
 	}
@@ -2218,7 +2218,7 @@ void __vm_get_stat(struct kvm_vm *vm, const char *stat_name, uint64_t *data,
 	size_t size_desc;
 	int i;
 
-	if (!vm->stats_fd) {
+	if (!vm->stats_desc) {
 		vm->stats_fd = vm_get_stats_fd(vm);
 		read_stats_header(vm->stats_fd, &vm->stats_header);
 		vm->stats_desc = read_stats_descriptors(vm->stats_fd,