I don't know if this helps but the same happens with me (qemu 1:2.8+dfsg-6), and I also tried removing all qemu packages and compiling 2.9.0 from scractch (downloaded the released 2.9.0 from the qemu site), and the problem persists.
It's also Windows 7 on a x86_64 system which is being emulated, and the memory leak only shows up when using the network. The command-line arguments are -monitor stdio \ -soundhw ac97 \ -machine accel=kvm \ -m 512 \ -hda /home/qemu-user/Windows_7_x64_HDA.img \ -boot once=c,menu=off \ -net nic,vlan=0 \ -net user,vlan=0 \ -net user,tftp=/home/qemu-user/win \ -net user,smb=/home/qemu-user/win \ -rtc base=localtime \ -name "Windows 7 x64" J.