Introduce Svirt test into virt-test/libvirt

Considering svirt is merged into libvirt, I Add a module in libvirt test for svirt. just like virsh_cmd in libvirt test.

  Add getter and setter of seclabel into vm_xml module.
  Add unittest for seclabel in libvirt_xml_unittest.
  Add utils_selinux in virttest.
  Add unittest for utils_selinux
  Add svirt test module in libvirt.

 libvirt/tests/cfg/svirt/svirt_attach_disk.cfg |   4 +
 libvirt/tests/src/svirt/__init__.py           |   0
 libvirt/tests/src/svirt/svirt_attach_disk.py  |   5 +
 virttest/libvirt_xml/vm_xml.py                |  76 ++++++++++
 virttest/libvirt_xml_unittest.py              |  21 +++
 virttest/utils_selinux.py                     | 205 ++++++++++++++++++++++++++
 virttest/utils_selinux_unittest.py            |  36 +++++
 7 files changed, 347 insertions(+)
 create mode 100644 libvirt/tests/cfg/svirt/svirt_attach_disk.cfg
 create mode 100644 libvirt/tests/src/svirt/__init__.py
 create mode 100644 libvirt/tests/src/svirt/svirt_attach_disk.py
 create mode 100644 virttest/utils_selinux.py
 create mode 100644 virttest/utils_selinux_unittest.py