Lockup with vhost network

After using Qemu in this configuration successfully for quite a while, I changed two things:
- moved the VM from a 8-core 4GHz host to a slower 2-core 1.6 Ghz machine
- upgraded qemu from 2.1 to 2.5

and almost immediately (in a couple hours) got hit with a vhost-related lockup.

QEMU command line is:

qemu-system-x86_64 -enable-kvm -daemonize -monitor unix:./monitor,server,nowait -cpu host -M q35 -balloon virtio -device virtio-scsi-pci,id=scsi -device scsi-hd,drive=hd -drive if=none,id=hd,cache=writeback,aio=native,format=raw,file=xxxx.img,discard=unmap,detect-zeroes=unmap -device virtio-net-pci,netdev=net0,mac=xxxx -netdev tap,vhost=on,id=net0,script=xxxx.sh -usbdevice tablet -smp 2 -m 512 -vnc xxxx:yz

VM was running fine, except no network traffic was passed from/to it. Shutting down the VM, it hung at "Will now halt." The QEMU process was unkillable, so the only choice was to sysrq-b the entire box.

dmesg with sysrq-w attached.