libxl: Refactor logic in domain post parse callback
    
    With the current control flow the post parse callback returned success
    right away for fully virtualized VMs. To allow adding additional checks
    into the post parse callback tweak the conditions so that the function
    doesn't return early except for error cases.
    
    To clarify the original piece of code borrow the wording from the commit
    message for the patch that introduced the code.