[BUGFIX] add _loadRealInstance in f:form ViewHelper

If you edit a form containing subproperties of your domain model,
f:form VH will add some hidden fields to identify your related
domain models (__identity).
But in case of @lazy annotations, these fields will not be
rendered because of a missing _loadRealInstance call 
in AbstractFormViewHelper object.

Releases 6.3, 6.2
Resolves: #60726
Change-Id: I67864e5b80752ff4350efabc72bd2b0a89d33bad
Reviewed-on: http://review.typo3.org/31934
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Frank Nägler <typo3@naegler.net>
Tested-by: Frank Nägler <typo3@naegler.net>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>