[xen] Set the "feature-rx-notify" flag for netfront devices

iPXE already sends RX notifications to the backend when needed, but
does not set the "feature-rx-notify" flag.  As of XenServer 6.5, this
flag is mandatory and omitting it will cause the backend to fail.

Fix by setting the "feature-rx-notify" flag, to inform the backend
that we will send notifications.

Reported-by: Shalom Bhooshi <shalom.bhooshi@citrix.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/netfront.c

 					  "feature-no-csum-offload" ) ) != 0 )
 		goto err_feature_no_csum_offload;
 
+	/* Inform backend that we will send notifications for RX requests */
+	if ( ( rc = netfront_write_flag ( netfront,
+					  "feature-rx-notify" ) ) != 0 )
+		goto err_feature_rx_notify;
+
 	/* Set state to Connected */
 	if ( ( rc = xenbus_set_state ( xendev, XenbusStateConnected ) ) != 0 ) {
 		DBGC ( netfront, "NETFRONT %s could not set state=\"%d\": %s\n",
  err_backend_wait:
 	netfront_reset ( netfront );
  err_set_state:
+	netfront_rm ( netfront, "feature-rx-notify" );
+ err_feature_rx_notify:
 	netfront_rm ( netfront, "feature-no-csum-offload" );
  err_feature_no_csum_offload:
 	netfront_rm ( netfront, "request-rx-copy" );
 	}
 
 	/* Delete flags */
+	netfront_rm ( netfront, "feature-rx-notify" );
 	netfront_rm ( netfront, "feature-no-csum-offload" );
 	netfront_rm ( netfront, "request-rx-copy" );