Adicionei a seguinte linha no
/etc/security/capability.conf:
cap_net_admin $user
cap_net_raw $user
E rodei:
sudo setcap cap_net_admin=ei /usr/bin/qemu-system-*
Reloguei e tudo funcionou.
[1] http://pacita.org/books/server-setup/output/htmlchunk/ch04.html
[2] http://blog.bravi.org/?p=912