stubdom: fix do_xen_hypercall error report

diff -r 70a43a82aaf1 tools/libxc/xc_minios.c
@@ -102,8 +102,8 @@
 	errno = -ret;
 	return -1;
     }
-    if (call.result < 0) {
-        errno = -call.result;
+    if ((long) call.result < 0) {
+        errno = - (long) call.result;
         return -1;
     }
     return call.result;