xenapi_utils: Adapt to enum cleanups

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>