libxl: fix compiler error introduced by commit ba25c214

libxl/libxl_conf.c: In function 'libxlDriverConfigNew':
libxl/libxl_conf.c:1560:30: error: 'log_level' may be used uninitialized
in this function [-Werror=maybe-uninitialized]

src/libxl/libxl_conf.c

@@ -1496,7 +1496,7 @@ libxlDriverConfigNew(void)
 {
     libxlDriverConfigPtr cfg;
     char *log_file = NULL;
-    xentoollog_level log_level;
+    xentoollog_level log_level = XTL_DEBUG;
     char ebuf[1024];
     unsigned int free_mem;