package/qemu: security bump to version 3.1.1

Fixes the following security issues:

CVE-2018-16872: A flaw was found in qemu Media Transfer Protocol (MTP).  The
code opening files in usb_mtp_get_object and usb_mtp_get_partial_object and
directories in usb_mtp_object_readdir doesn't consider that the underlying
filesystem may have changed since the time lstat(2) was called in
usb_mtp_object_alloc, a classical TOCTTOU problem.  An attacker with write
access to the host filesystem shared with a guest can use this property to
navigate the host filesystem in the context of the QEMU process and read any
file the QEMU process has access to.  Access to the filesystem may be local
or via a network share protocol such as CIFS.

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