vnc: Fix incorrect attempt to clear a flag

In vnc.c in pixel_format_message, the code tries to clear the
QEMU_ALLOCATED_FLAG from the client display surface, however
it uses the wrong operator and ends up enabling all other
flags. Most notably this enables the big endian flag and
causes some chaos.

Signed-off-by: Brian Kress <kressb@moose.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7022 c046a42c-6fe2-441c-8c8c-71466251a162