Fix xapi xm-tests.

Fix xapi xm-tests.

There were a couple of small bugs in the xapi xm-test:
o outdated XenAPI calls were removed from testcase
  (02_xapi-vbd_basic)
o minor problem with XendLocalStorageRepository
  is fixed (missed list_images() function - which
  is moved from the XenQCoWStroageRepo to the common
  base class XendStorageRepository)
  which was detected running 02_xapi-vbd_basic.
o XenAPI session handling and connecting is fixed.
o 03_xapi-network_pos was rewritten and now uses
  XenAPI.

Signed-off-by: Andreas Florath <xen@flonatel.org>