reds: Simplify vdi_port_read_buf_process
reds: Simplify vdi_port_read_buf_process

Reuse and handle the return value from agent_msg_filter_process_data

Acked-by: Frediano Ziglio <55d48b080b2e443e395cde84d2c83b135a4ff48e@redhat.com>

fgouget/spice,fgouget/spice,fgouget/spice,fgouget/spice