qapi: More robust conditions for when labels are needed

We were using regular expressions to see if ret included
any earlier text that emitted a 'goto out;' line, to decide
whether we needed to output an 'out:' label.  But this is
fragile, if the ret text can possibly combine more than one
generated function body, where the first function used a
goto but the second does not.  Change the code to just check
for the known conditions which cause an error check to be
needed.  Besides, it's slightly more efficient to use plain
checks than regular expression searching.

No change to generated code.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1445898903-12082-4-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
