qapi: Make BlockdevOptions doc example closer to reality

qapi: Make BlockdevOptions doc example closer to reality

Although we don't want to repeat the entire BlockdevOptions
QMP command in the example, it helps if we aren't needlessly
diverging (the initial example was written before we had
committed the actual QMP interface).  Use names that match what
is found in qapi/block-core.json, such as '*read-only' rather
than 'readonly', or 'BlockdevRef' rather than 'BlockRef'.

For the simple union example, invent BlockdevOptionsSimple so
that later text is unambiguous which of the two union forms is
meant (telling the user to refer back to two 'BlockdevOptions'
wasn't nice, and QMP has only the flat union form).

Also, mention that the discriminator of a flat union is
non-optional.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1458254921-17042-14-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>