qapi: use valid JSON in schema

* qapi-schema.json: JSON doesn't allow trailing commas.
Signed-off-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

qapi-schema.json

@@ -1600,7 +1600,7 @@
 ##
 { 'union': 'BlockdevAction',
   'data': {
-       'blockdev-snapshot-sync': 'BlockdevSnapshot',
+       'blockdev-snapshot-sync': 'BlockdevSnapshot'
    } }
 
 ##