qemu: security bump to version 2.8.1.1

qemu: security bump to version 2.8.1.1

Fixes the following security issues and adds a number of other bigfixes:

2.8.1: Changelog:
https://lists.gnu.org/archive/html/qemu-devel/2017-03/msg06332.html

CVE-2017-2615 - display: cirrus: oob access while doing bitblt copy backward
mode

CVE-2017-2620 - display: cirrus: out-of-bounds access issue while in
cirrus_bitblt_cputovideo

CVE-2017-2630 - nbd: oob stack write in client routine drop_sync

2.8.1.1 Changelog:
https://lists.gnu.org/archive/html/qemu-devel/2017-04/msg03460.html

CVE-2017-7471 - 9p: virtfs allows guest to change filesystem attributes on
host

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>