This commit fixes breaking change [0].
Long driver names like 'nova.virt.libvirt.libvirtDriver' are
no longer available and 'libvirt.libvirtDriver' should be used
instead.
Reference:
[0] https://review.openstack.org/309504
Change-Id: I27a1b75b921c7401bc8614caadfd1e09e7dd5d65
Closes-Bug: 1574990