Generalize PCI device as host device

There are some virtual devices are virtualized from physical PCI devices and
have some common characteristics as PCI devices but they may be not real PCI
devices e.g. vGPU, vTPM, vFPGA. If we can generalize PCI devices by
abstracting some common concepts in a top object, we can re-use most of
those exiting PCI code to handle those non-PCI virtual devices. It will be
helpful to make it easier to introduce new devices to OpenStack.