The conditions to detect overflow in sub operations was wrong.

This patch is necessary to boot Tru64.
Signed-off-by: NTristan Gingold <gingold@adacore.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7073 c046a42c-6fe2-441c-8c8c-71466251a162