Increase size of buffer for xend response

On systems with many pcpus, the sexpr returned by xend can be quite
large for dom0 when it is configured to have #vcpus = #pcpus (default).
E.g. on a 80 pcpu system, where dom0 had 80 vcpus, the sexpr details
for dom0 was 73817 bytes!  Increase maximum buffer size to 256k.

src/xen/xend_internal.c

@@ -56,7 +56,7 @@
  * The number of Xen scheduler parameters
  */
 
-#define XEND_RCV_BUF_MAX_LEN 65536
+#define XEND_RCV_BUF_MAX_LEN (256 * 1024)
 
 static int
 virDomainXMLDevID(virDomainPtr domain,