io: get rid of bounce buffering in websock write path

Currently most outbound I/O on the websock channel gets copied into the
rawoutput buffer, and then immediately copied again into the encoutput
buffer, with a header prepended. Now that qio_channel_websock_encode
accepts a struct iovec, we can trivially remove this bounce buffering
and write directly to encoutput.

In doing so, we also now correctly validate the encoutput size against
the QIO_CHANNEL_WEBSOCK_MAX_BUFFER limit.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
