churn(libvirt): Rename host from dock0 to coreos0

Makes it sound less dockerish which isn't even a part of this tutorial.
churn(libvirt): Move image location under /var

Using /usr/src for libvirt images isn't standard, use something that is
a little bit more typical.
fix(libvirt): Update config for modern qemu times.

These days 'qemu-kvm' is preferred over 'kvm' and don't bother locking
the VM to the old 0.15 hardware definition.
feat(libvirt): Rework documentation around using config drive.
fix(libvirt): Add missing links
feat(openstack): Add note on using config drive.
Merge pull request #159 from marineam/libvirt

Libvirt and OpenStack config drive