[pxe] Fix broadcast transmissions via PXENV_UDP_WRITE

pxenv_udp_write() needs to specify a net device, otherwise ipv4 is
unable to route broadcast transmissions.

src/interface/pxe/pxe_udp.c

 	struct xfer_metadata meta = {
 		.src = ( struct sockaddr * ) &pxe_udp.local,
 		.dest = ( struct sockaddr * ) &dest,
+		.netdev = pxe_netdev,
 	};
 	size_t len;
 	struct io_buffer *iobuf;