Placement model for passthrough devices

Deployers want to be able to attach accelerators and other devices to their
VMs. Today in Nova this is possible only in very restrictive circumstances.
For PCI devices as an example, the primary issues are:

* the special way in which PCI devices are currently inventoried
* the special way to schedule VMs that request pci passthrough, which doesn't
  rely on Placement to provide the correct allocation candidates, but on the
  ``PciPassthroughFilter`` to filter out incapable nodes
* no quotas on PCI devices

To address these issues, this blueprint proposes a Placement model for devices
(PCI or other kinds). This consists of a common infrastructure shared among all the
drivers and specific implementations for the libvirt and powervm drivers. The
implementation for 2 drivers paves the way for implementing it in any other.

The blueprint also includes an overhaul of device allocation/deallocation
to tie in with the Placement model.