libxl: Correct cast to virDomainDiskDiscard enum.

This was converted to a typedef in 5a2bd4c9 "conf: more enum
cleanups in "src/conf/domain_conf.h"" causing:
    libxl/libxl_conf.c: In function 'libxlDiskSetDiscard':
    libxl/libxl_conf.c:724:19: error: conversion to incomplete type
Signed-off-by: NIan Campbell <ian.campbell@citrix.com>

src/libxl/libxl_conf.c

@@ -721,7 +721,7 @@ libxlDiskSetDiscard(libxl_device_disk *x_disk, int discard)
     if (!x_disk->readwrite)
         return 0;
 #if defined(LIBXL_HAVE_LIBXL_DEVICE_DISK_DISCARD_ENABLE)
-    switch ((enum virDomainDiskDiscard)discard) {
+    switch ((virDomainDiskDiscard)discard) {
     case VIR_DOMAIN_DISK_DISCARD_DEFAULT:
     case VIR_DOMAIN_DISK_DISCARD_LAST:
         break;