Add getter and setter of seclabel into vm_xml module.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Add seclabel related unittest into libvirt_xml_unittest

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Add utils_selinux in virttest.

changes from v1:
(1).use getfattr to replace ls -Z
(2).use /proc//attr/current to replace ps -Z
(3).remove some error type seems unnecssary.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>
Add unittest for utils_selinux.

Signed-off-by: yangdongsheng <yangds.fnst@cn.fujitsu.com>