Use disk_bus and device_type in attaching volumes

This patch enables users to pass disk_bus and device_type parameters to
attach action api call of the extended-volumes plugin (v3 API only).

It also implements this functionality inside the libvirt driver.

Implements blueprint: use-new-bdm-format-in-attach

Change-Id: I7895974def7057fbaddf2b3715a8b72a76f88f3e