hv_xen: Prepare for unit tests, remove {static,class}method

Unit tests will have to specify a custom Xen configuration directory (a
temporary directory). To this end “hv_xen.XenHypervisor” and its two
derived classes are modified to have fewer static and class methods. A
new private attribute for the configuration directory is also added (to
be used in test code).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>