conf: avoid looking up network port that doesn't exist

If the hypervisor driver has not yet created the network port, the
portid field will be "00000000-0000-0000-0000-000000000000".

If a failure occurs during early VM startup, the hypervisor driver may
none the less try to release the network port, resulting in an
undesirable warning:

2019-09-12 13:17:42.349+0000: 16544: error :
virNetworkObjLookupPort:1679 : network port not found: Network port with
UUID 00000000-0000-0000-0000-000000000000 does not exist

By checking if the portid UUID is valid, we can avoid polluting the logs
in this way.
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

src/conf/domain_conf.c

@@ -30994,6 +30994,12 @@ virDomainNetReleaseActualDevice(virConnectPtr conn,
     virNetworkPortPtr port = NULL;
     int ret = -1;
 
+    /* Port might not exist if a failure occurred during VM startup */
+    if (!virUUIDIsValid(iface->data.network.portid)) {
+        ret = 0;
+        goto cleanup;
+    }
+
     if (!(net = virNetworkLookupByName(conn, iface->data.network.name)))
         goto cleanup;