vfio/pci: Add emulated PCI IDs

Specifying an emulated PCI vendor/device ID can be useful for testing
various quirk paths, even though the behavior and functionality of
the device with bogus IDs is fully unsupportable.  We need to use a
uint32_t for the vendor/device IDs, even though the registers
themselves are only 16-bit in order to be able to determine whether
the value is valid and user set.

The same support is added for subsystem vendor/device ID, though these
have the possibility of being useful and supported for more than a
testing tool.  An emulated platform might want to impose their own
subsystem IDs or at least hide the physical subsystem ID.  Windows
guests will often reinstall drivers due to a change in subsystem IDs,
something that VM users may want to avoid.  Of course careful
attention would be required to ensure that guest drivers do not rely
on the subsystem ID as a basis for device driver quirks.

All of these options are added using the standard experimental option
prefix and should not be considered stable.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
