libvirt driver capable of generating device names

Implement the method from the base driver class to let libvirt
provide device names if the manager requests them.

This allows to boot an instance without having to care about
device names and delegating that task to the compute driver.

blueprint: improve-block-device-handling

Change-Id: I2b4f2ea6a4d2e4133e0ba12edabaa754744b8fa4