char-device: fix usage of free/unref on WriteBuffer
char-device: fix usage of free/unref on WriteBuffer

There are places were the could should definetly free the
SpiceCharDeviceWriteBuffer and places that it should only unref it. The
current use of spice_char_device_write_buffer_free was missleading.

This patch creates the spice_char_device_write_buffer_unref and properly
call these two functions.

Related: https://bugs.freedesktop.org/show_bug.cgi?id=91350

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