qobject: qobject_from_jsonv() is dangerous, hide it away

qobject: qobject_from_jsonv() is dangerous, hide it away

qobject_from_jsonv() takes ownership of %p arguments.  On failure, we
can't generally know whether we failed before or after %p, so
ownership becomes indeterminate.  To avoid leaks, callers passing %p
must terminate on error, e.g. by passing &error_abort.  Trap for the
unwary; document and give the function internal linkage.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180806065344.7103-11-armbru@redhat.com>