Fix crash when multiple event callbacks were registered

CVE-2013-2230

Don't overwrite the callback ID returned by
virDomainEventStateRegisterID in ret by 0.

Introduced by abf75aea.

src/qemu/qemu_driver.c

@@ -10043,9 +10043,7 @@ qemuConnectDomainEventRegisterAny(virConnectPtr conn,
                                       driver->domainEventState,
                                       dom, eventID,
                                       callback, opaque, freecb, &ret) < 0)
-        goto cleanup;
-
-    ret = 0;
+        ret = -1;
 
 cleanup:
     return ret;