vnc-tight: fix regression with libxenstore

commit 095497ff added thread local storage for the color counting
palette. Unfortunately, a VncPalette is about 7kB on a x86_64 system.
This memory is reserved from the stack of every thread and it
exhausted the stack space of a libxenstore thread.

Fix this by allocating memory only for the VNC encoding thread.

Fixes: 095497ff
Reported-by: Juergen Gross <jgross@suse.com>
Tested-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Peter Lieven <pl@kamp.de>
Message-id: 1468575911-20656-1-git-send-email-pl@kamp.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>