manager: fix assumption of child connections with autoconnect=no

During startup, when a link is detected (enp0s25 in the example below)
we try to create also virtual devices (ipip1) on it through
system_create_virtual_device(), however this realizes only devices for
connections which can autoactivate.

To support the assumption of child devices with autoconnect=no, we
should take in consideration in retry_connections_for_parent_device()
only connections for which the link does not exist, and let existing
links be handled by platform_link_added(), which also realizes them.

Reproducer:
 $ nmcli c add type ip-tunnel ifname ipip1 con-name ipip1+ autoconnect no \
               mode ipip remote 172.25.16.1 dev enp0s25 ip4 1.2.3.4/31
 $ nmcli c up ipip1+
 $ systemctl restart NetworkManager

Result:
 * before: ipip1+ is not assumed, ipip1 is not present in 'nmcli d' output
 * after:  ipip1+ is assumed, ipip1 detected

1 files changed, 10 insertions, 2 deletions

diff --git a/src/nm-manager.c b/src/nm-manager.c
index 6d7f96ccbf..409079a3e4 100644
--- a/src/nm-manager.c
+++ b/src/nm-manager.c
@@ -1136,11 +1136,19 @@ retry_connections_for_parent_device (NMManager *self, NMDevice *device)
 	connections = nm_settings_get_connections (priv->settings);
 	for (iter = connections; iter; iter = g_slist_next (iter)) {
 		NMConnection *candidate = iter->data;
+		gs_free_error GError *error = NULL;
+		gs_free char *ifname = NULL;
 		NMDevice *parent;
 
 		parent = find_parent_device_for_connection (self, candidate, NULL);
-		if (parent == device)
-			connection_changed (priv->settings, candidate, self);
+		if (parent == device) {
+			/* Only try to activate devices that don't already exist */
+			ifname = nm_manager_get_connection_iface (self, candidate, &parent, &error);
+			if (ifname) {
+				if (!nm_platform_link_get_by_ifname (NM_PLATFORM_GET, ifname))
+					connection_changed (priv->settings, candidate, self);
+			}
+		}
 	}
 
 	g_slist_free (connections);