io: get rid of qio_channel_websock_encode helper method

The qio_channel_websock_encode method is only used in one place,
everything else calls qio_channel_websock_encode_buffer directly.
It can also be pushed up a level into the qio_channel_websock_writev
method, since every other caller of qio_channel_websock_write_wire
has already filled encoutput.

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