Add unit test for XenAPI plugins

No unit test for XenAPI plugins currently. Unit test will help to find issues earlier and easier. XenAPI plugins run in XenServer's dom0 which currently only has py2.4; so these plugins are developed with py2.4. but usually the unit test will be run with py2.7 or py3.4. For these tests, we plan to cover it by using py2.7, so the unit tests should be compatible with py2.4 and py2.7. And all of these new tests will be excluded from py34 testing.

All of the plugins under the following path will be covered by this spec:
    plugins/xenserver/xenapi/etc/xapi.d/plugins/