qapi: Comment version info in TransactionAction

Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NMarkus Armbruster <armbru@redhat.com>
Message-id: 1418899027-8445-2-git-send-email-famz@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

qapi-schema.json

@@ -1254,6 +1254,12 @@
 #
 # A discriminated record of operations that can be performed with
 # @transaction.
+#
+# Since 1.1
+#
+# drive-backup since 1.6
+# abort since 1.6
+# blockdev-snapshot-internal-sync since 1.7
 ##
 { 'union': 'TransactionAction',
   'data': {