vbox_tmpl.c: don't leak a domain pointer upon failure to create

* src/vbox/vbox_tmpl.c (vboxDomainCreateXML): "Unref" the domain
upon failure.  Patch by Matthias Bolte.

src/vbox/vbox_tmpl.c

@@ -1004,6 +1004,7 @@ static virDomainPtr vboxDomainCreateXML(virConnectPtr conn, const char *xml,
 
     if (vboxDomainCreate(dom) < 0) {
         vboxDomainUndefine(dom);
+        virUnrefDomain(dom);
         return NULL;
     }