I wish I could tell libvirtd to disable usb for specific hosts.
I understand it's always passed right now to enable hotplugging, but
I'd like to be able to turn it off if I never want hotplugging on
a host.

Hi Martin,
Your talking about dropping kvm/qemu's -usb option?
 -- Guido

also sprach Guido Günther <agx@sigxcpu.org> [2009.05.08.1313 +0200]:

Yes, although I have since learnt that kvm is planning to move to
make it default. In any case, it would be nice to be able to disable
any USB subsystem for VMs started by libvirt.