xenapi_utils: Adapt to enum cleanups
    
    It was just very recently that we transfered from:
    
      enum virSomeEnumName{
          ...
      };
    
    to:
      typedef enum {
          ...
      } virSomeEnumName;
    
    This change requires some code adaptation, which wasn't done for
    xenapi driver. With this fix we are able to build again.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>