qapi: Rename class QAPISchema to QAPISchemaParser

I want to name a new class QAPISchema.

While there, make it a new-style class.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1442401589-24189-2-git-send-email-armbru@redhat.com>