CP-8358: PCI subsystem device/vendor ID handling

These patches add the PCI subsystem device/vendor ID/name to PCI objects, parsing them from lspci output on xapi startup.

Support is also added for specifying a physical subsystem device ID in vGPU config files; xapi will take this into account when determining whether a vGPU configuration is supported on a discovered PCI device.

If a config file does not specify a physical subsystem device ID, xapi will interpret this to mean that only the physical device ID is required to identify a device which will support the vGPU configuration.
> It returns "", with a 0 exit code.

Ok, in that case I think xapi would come to the conclusion that there were no PCI devices installed.