selftests: kvm: avoid uninitialized variable warning

The variable in practice will never be uninitialized, because the
loop will always go through at least one iteration.

In case it would not, make vcpu_get_cpuid report an assertion
failure.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tools/testing/selftests/kvm/lib/x86_64/processor.c

@@ -720,7 +720,8 @@ struct kvm_cpuid2 *vcpu_get_cpuid(struct kvm_vm *vm, uint32_t vcpuid)
 {
 	struct vcpu *vcpu = vcpu_find(vm, vcpuid);
 	struct kvm_cpuid2 *cpuid;
-	int rc, max_ent;
+	int max_ent;
+	int rc = -1;
 
 	TEST_ASSERT(vcpu != NULL, "vcpu not found, vcpuid: %u", vcpuid);