FEATURE: PCI-3773 aio=native and virtio-blk-dataplane enablement

This commit introduces following configuration options to nova.conf

* libvirt_dataplane = (True|False)
* libvirt_io_native = (True|False)

and related functionality.

All changes are backed by tests.
Merge pull request #38 from ytsarev/yut-dataplane

FEATURE: PCI-3773 aio=native and virtio-blk-dataplane enablement