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.
