nbd: convert to use qio_channel_yield

nbd: convert to use qio_channel_yield

In the client, read the reply headers from a coroutine, switching the
read side between the "read header" coroutine and the I/O coroutine that
reads the body of the reply.

In the server, if the server can read more requests it will create a new
"read request" coroutine as soon as a request has been read.  Otherwise,
the new coroutine is created in nbd_request_put.

Reviewed-by: Stefan Hajnoczi <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Reviewed-by: Daniel P. Berrange <[email protected]>
Message-id: 20170213135235[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>