qapi: restrict allow-oob value to be "true"

It was missed in the first version of OOB series.  We should check this
to make sure we throw the right error when fault value is passed in.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180326063901.27425-5-peterx@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

scripts/qapi/common.py

@@ -872,7 +872,7 @@ def check_keys(expr_elem, meta, required, optional=[]):
             raise QAPISemError(info,
                                "'%s' of %s '%s' should only use false value"
                                % (key, meta, name))
-        if key == 'boxed' and value is not True:
+        if (key == 'boxed' or key == 'allow-oob') and value is not True:
             raise QAPISemError(info,
                                "'%s' of %s '%s' should only use true value"
                                % (key, meta, name))