virttest.libvirt_xml: Add class for cputune entry

1) Use a dedicated class VMCPUTune to manipulate `<cputune>` tag
   instead of using a tag string and a lot of attributes which is
   not intuitive when using.

2) Add a simple getter setter delter unittest for this class.

3) Fixed two `<string> is not <string>` to `<string> != <string>`.
   The first one is identity test and will not alway get `False`
   even if two strings are equal.

Signed-off-by: Hao Liu <hliu@redhat.com>