- update XMLVMachineData class to reflect mandatory status of
  <vmdisk> configuration setting in XML. Previously the <vmdisk>
  element was optional and it was possible to construct the
  VMachine object without arguments. Now the <vmdisk> element
  is mandatory and thus the object may no longer be constructed
  without argument.
- added support for post hook commands as requested in (bnc #776535)
- DB: add documentation about new post hook commands
- DB: rebuild documentation