vnc: fix a memory leak in threaded vnc server

VncJobQueue's buffer is intended to be used for
as the output buffer for all operations in this queue,
but unfortunatly.

vnc_async_encoding_start() is in charge of setting this
buffer as the current output buffer, but
vnc_async_encoding_end() was not writting the changes back
to VncJobQueue, resulting in a big and ugly memleak.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
