qapi: Prepare for requiring more complete documentation

We currently neglect to check all enumeration values, common members
of object types and members of alternate types are documented.
Unsurprisingly, many aren't.

Add the necessary plumbing to find undocumented ones, except for
variant members of object types.  Don't enforce anything just yet, but
connect each QAPIDoc.ArgSection to its QAPISchemaMember.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1489582656-31133-15-git-send-email-armbru@redhat.com>
