util: remove error log from stubs of virNetlinkEventServiceStart|Stop

These two functions are called from main() on all platforms, and
always return success on platforms that don't support libnl. They
still log an error message, though, which doesn't make sense - they
should just be NOPs on those platforms. (Per a suggestion during
review, I've turned the logs into debug messages rather than removing
them completely).

src/util/virnetlink.c

@@ -561,7 +561,7 @@ int virNetlinkCommand(struct nl_msg *nl_msg ATTRIBUTE_UNUSED,
  */
 int virNetlinkEventServiceStop(void)
 {
-    netlinkError(VIR_ERR_INTERNAL_ERROR, "%s", _(unsupported));
+    VIR_DEBUG("%s", _(unsupported));
     return 0;
 }
 
@@ -571,7 +571,7 @@ int virNetlinkEventServiceStop(void)
  */
 int virNetlinkEventServiceStart(void)
 {
-    netlinkError(VIR_ERR_INTERNAL_ERROR, "%s", _(unsupported));
+    VIR_DEBUG("%s", _(unsupported));
     return 0;
 }