CVE-2017-18030 already adressed earlier with the v2.8.1 import

The qemu update versioned 1:2.8+dfsg-4 in unstable did include as patch
the 2.8.1 upstream stable/bugfix release including the commit

https://git.qemu.org/?p=qemu.git;a=commitdiff;h=f153b563f8cf121aebf5a2fff5f0110faf58ccb3

Adjust the fixing version accordingly.