manager: keep device if realize() fails

system_create_virtual_device() first creates the device (unrealized)
and then, if there a connection for the device with autoconnect=yes,
creates the backing resources. If this last step fails the device
should continue to exist, even if in an unrealized state.

https://bugzilla.redhat.com/show_bug.cgi?id=1686634

src/nm-manager.c

@@ -2032,7 +2032,6 @@ system_create_virtual_device (NMManager *self, NMConnection *connection)
 			         "couldn't create the device: %s",
 			         error->message);
 			g_error_free (error);
-			remove_device (self, device, FALSE);
 			return NULL;
 		}