qapi: Fix detection of bogus member documentation

check_definition_doc() checks for member documentation without a
matching member.  It laboriously second-guesses what members
QAPISchema._def_exprs() will create.  That's a stupid game.

Move the check into QAPISchema.check(), where the members are known.
Delegate the actual checking to new QAPIDoc.check().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-38-git-send-email-armbru@redhat.com>
