util: relax virNetDevSetCoalesce() stub

Currently, virNetDevSetCoalesce() stub is always returning error. As
it's used by virNetDevTapCreateInBridgePort(), it essentially breaks
bridged networking if coalesce is not supported.

To make it work, relax the stub to trigger error only when its
coalesce argument is not NULL, otherwise report success.

src/util/virnetdev.c

@@ -3155,8 +3155,11 @@ int virNetDevSetCoalesce(const char *ifname,
 }
 # else
 int virNetDevSetCoalesce(const char *ifname,
-                         virNetDevCoalescePtr coalesce ATTRIBUTE_UNUSED)
+                         virNetDevCoalescePtr coalesce)
 {
+    if (!coalesce)
+        return 0;
+
     virReportSystemError(ENOSYS,
                          _("Cannot set coalesce info on interface '%s'"),
                          ifname);
@@ -3213,8 +3216,11 @@ virNetDevGetFeatures(const char *ifname ATTRIBUTE_UNUSED,
 }
 
 int virNetDevSetCoalesce(const char *ifname,
-                         virNetDevCoalescePtr coalesce ATTRIBUTE_UNUSED)
+                         virNetDevCoalescePtr coalesce)
 {
+    if (!coalesce)
+        return 0;
+
     virReportSystemError(ENOSYS,
                          _("Cannot set coalesce info on interface '%s'"),
                          ifname);