Free up memballoon def.

Forgetting to do this was causing a memory leak.
Signed-off-by: NChris Lalancette <clalance@redhat.com>

src/conf/domain_conf.c

@@ -768,6 +768,8 @@ void virDomainDefFree(virDomainDefPtr def)
 
     virDomainWatchdogDefFree(def->watchdog);
 
+    virDomainMemballoonDefFree(def->memballoon);
+
     virSecurityLabelDefFree(def);
 
     virCPUDefFree(def->cpu);