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>

tests/qapi-schema/test-qapi.py

@@ -12,10 +12,11 @@
 
 from qapi import *
 from pprint import pprint
+import os
 import sys
 
 try:
-    exprs = parse_schema(sys.stdin)
+    exprs = parse_schema(sys.argv[1])
 except SystemExit:
     raise