Add a helper for query field checks

… and also use it to simplify 'needsLiveData'. Additionally, add an
explicit export list to Ganeti.Query.Types, since otherwise we'd
(re)export all imported symbols.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>