qobject: Drop QObject_HEAD

qobject: Drop QObject_HEAD

QObject_HEAD is a macro expanding into the common part of structs that
are sub-types of QObject.  It's always been just QObject base, and
unlikely to change.  Drop the macro, because the code is clearer with
out it.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1444918537-18107-2-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>