We're seeing this on x86-64 Wheezy with the libvirt backports version (currently 1.2.9-9~bpo70+1), too. A virt-manager client was attached several days, ramping up libvirtd memory usage (RES) to >8GB.
The workaround (restarting libvirtd) helped. -- PGP-Key 0xD40E0E7A
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