qapi.py: Fix schema parser to check syntax systematically

Fixes at least the following parser bugs:

* accepts any token in place of a colon

* treats comma as optional

* crashes when closing braces or brackets are missing

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 1374939721-7876-7-git-send-email-armbru@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
