virDomainDefParseXML: Free @tmp

When parsing <ioapic> feature we're using @tmp to store some
temporary string but never free it.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/conf/domain_conf.c

@@ -17892,6 +17892,7 @@ virDomainDefParseXML(xmlDocPtr xml,
                 }
                 def->ioapic = value;
                 def->features[val] = VIR_TRISTATE_SWITCH_ON;
+                VIR_FREE(tmp);
             }
             ctxt->node = node;
             break;