- renames the XenManagedDomain.py file to XenAPIDomain.py, since this
name better reflects its functionality/purpose
- adds domain tracking to the XenAPIDomain class so that xend-managed
domains can be deleted in an 'atexit' handler upon test case termination
- adds one basic xapi-related test which is part of the grouptests
'xapi'
- refactors the vtpm-related test using xen-api and adds it to the
grouptest 'xapi'
- adds documentation to the README for how to configure xm and xend to
use XML-RPC or Xen-API for communication

Signed-off-by: Stefan Berger <stefanb@us.ibm.com>