From libguestfs ≥ 1.19.24 you can, if you want, use libvirt to manage the libguestfs appliance. To enable this, set the environment variable:
export LIBGUESTFS_ATTACH_METHOD=libvirt
and (hopefully) any libguestfs program, guestfish, virt tool etc will just work as normal. As well as the absolute latest libguestfs you’ll also need libvirt ≥ 0.9.13 and a recent qemu.
What’s the point of this change? There are a couple of advantages. Firstly libvirt confines guests using sVirt mandatory access control (using SELinux or AppArmor). This will [it’s not working right now] provide additional defences against rogue disk images subverting the appliance or qemu.
Secondly libvirt already does disk hotplugging, and it will allow us to add this easily to libguestfs.
Richard WM Jones 
What is currently in use instead of libvirt? (<= f17) I'm confused…
libguestfs can either launch qemu/KVM directly or can run the appliance via libvirt, depending on how it has been configured and what environment variables are other things are set.