qapi: Use an explicit input file

Use an explicit input file on the command-line instead of reading from standard
input.

It also outputs the proper file name when there's an error.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/tests/qapi-schema/funny-char.err b/tests/qapi-schema/funny-char.err
index d3dd293..bfc890c 100644
--- a/tests/qapi-schema/funny-char.err
+++ b/tests/qapi-schema/funny-char.err
@@ -1 +1 @@
-<stdin>:2:36: Stray ";"
+tests/qapi-schema/funny-char.json:2:36: Stray ";"