Qemu networking broken after upgrade to Ubuntu 15.04?

I remember I went through a bit of puzzles to get my qemu guest up and running, so that I could connect to my guest from the host, through setting up a tap interface.
However, after the upgrade to Ubuntu 15.04, this is no longer working. I can get my guest to network in user mode, but that's it, so I suspect something changed in the new qemu 2.2 or elsewhere in Ubuntu networking.

I am starting my guest through:

qemu-system-x86_64 -enable-kvm [...] -net nic,model=e1000,vlan=0 -net tap,ifname=tap0,vlan=0,script=no

Any help in troubleshooting is appreciated...