tools/libxl: avoid comparing an unsigned int to -1

Signed-off-by: Koushik Chakravarty <koushik.chakravarty@citrix.com>
CC: Ian Jackson <ian.jackson@eu.citrix.com>
CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>

tools/libxl/libxl_json.c

@@ -1013,7 +1013,7 @@ out:
 yajl_gen_status libxl__uint64_gen_json(yajl_gen hand, uint64_t val)
 {
     char *num;
-    unsigned int len;
+    int len;
     yajl_gen_status s;