qom: Allow clearing of a Link property

By passing in "" to object_property_set_link.

The lead user of this is the QDEV GPIO framework which will implement
GPIO disconnects via an "unlink".  GPIO disconnection is used by
qtest's irq_intercept_out command.
Reviewed-by: NAlexander Graf <agraf@suse.de>
Signed-off-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

qom/object.c

@@ -872,9 +872,13 @@ char *object_property_get_str(Object *obj, const char *name,
 void object_property_set_link(Object *obj, Object *value,
                               const char *name, Error **errp)
 {
+    if (value) {
         gchar *path = object_get_canonical_path(value);
         object_property_set_str(obj, path, name, errp);
         g_free(path);
+    } else {
+        object_property_set_str(obj, "", name, errp);
+    }
 }
 
 Object *object_property_get_link(Object *obj, const char *name,