Cyborg NVIDIA GPU driver support vGPU management

GPU devices can provide supercomputing capabilities, and can replace the CPU
to provide users with more efficient computing power at a lower cost. GPU cloud
servers have great value in the following application scenarios, including:
video encoding and decoding, scientific research and artificial intelligence
(deep learning, machine learning).

In the OpenStack ecosystem, users can now use nova to pass gpu resources to
guest by two methods:

* Pass the GPU hardware to the guest (PCI pass-through).

* Pass the Mediated Device(vGPU) to the guest.

With the long-term goal that cyborg will manage heterogeneous accelerators
including GPUs, cyborg needs to support GPU management and integrate with nova
to provide users with gpu resources allocation in the aforementioned methods.
The existing cyborg gpu driver, NVIDIAGPUDriver, has supported the first method
(PCI pass-through), while the second method is not yet supported.