I've observed a memory leak in libvirt-bin version 1.1.4-2~bpo70+1 (current in backports).
While virt-manager is connected, periodically, the RES attribute (as reported by htop) will monotonically increase by 4. While disconnected, the value does not increase.
I do not know if it is related or not, but in my environment, libvirt is configured with server certificates stored in /etc/PKI (in the default location) and launched with /usr/sbin/libvirtd -d -l by init script. Again, the value only inccrements when a client is connected. It occurs with both local and ssl clients. I've observed it climbing (slowly) from .1% usage at launch to (currently after about 2 hours) .7% and I've seen it as high as 30% where it began to cause memory pressure and light swapping..
The process never releases the data, even after a client disconnects.
Thank you also to whomever maintains libvirt in backports. I appreciate the newer version :)
If anyone else is effected by this, a simple workaround is to simply schedule a restart of libvirt-bin process every 6-24 hours. Or to only connect to the daemon as needed.