Use libgfapi for compute access to cinder volumes

I've verified this works, in as far as everything gets passed correctly through to nova/libvirt/qemu in order to access a cinder volume via libgfapi when doing 'nova volume-attach'.  However there's a bug somewhere in qemu/libgfapi that causes qemu to hang the VM.  I'm still trying to run that down and file appropriate bugs.

For astapor purposes though, this does what we want.