qapi-schema: Fix up misleading specification of netdev_add

It doesn't take a 'props' argument, let alone one in the format
"NAME=VALUE,..."

The bogus arguments specification doesn't matter due to 'gen': false.
Clean it up to be incomplete rather than wrong, and document the
incompleteness.

While there, improve netdev_add usage example in the manual: add a
device option to show how it's done.

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: <1442401589-24189-24-git-send-email-armbru@redhat.com>