Establish base for testing Xen hypervisor abstraction

There are two separate Xen hypervisors (HVM and PVM), as well as two
different Xen commands (xl and xm). This already provides four different
combinations and future changes might bring even more. For this reason
the test classes for the Xen hypervisor are not defined manually, but
rather generated at module load time. Doing so makes it possible to keep
using the standard unit test infrastructure provided by the “unittest”
module.

Using “super”, which we try to avoid in other places, is necessary to
call functions provided by other classes.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>