util: moving 'type' argument to avoid issues with mount() syscall.

This commit fixes a mount call inside virgroup.c file. The NULL value
into 'type' argument is causing a valgrind issue. See commit 794b576c
for more details. The best approach to fix it is moving NULL to "none"
filesytem.
Signed-off-by: NJulio Faracco <jcfaracco@gmail.com>

src/util/vircgroup.c

@@ -3962,7 +3962,7 @@ virCgroupBindMount(virCgroupPtr group, const char *oldroot,
                 goto cleanup;
             }
 
-            if (mount(src, group->controllers[i].mountPoint, NULL, MS_BIND,
+            if (mount(src, group->controllers[i].mountPoint, "none", MS_BIND,
                       NULL) < 0) {
                 virReportSystemError(errno,
                                      _("Failed to bind cgroup '%s' on '%s'"),