vfio/pci: Introduce x-pci-class-code option

Introduce x-pci-class-code option to allow users to override PCI class
code of a device, similar to the existing x-pci-vendor-id option. Only
the lower 24 bits of this option are used, though a uint32 is used here
for determining whether the value is valid and set by user.

Additionally, to ensure VGA ranges are only exposed on VGA devices,
pci_register_vga() is now called in vfio_pci_config_setup(), after
the class code override is completed.

This is mainly intended for IGD devices that expose themselves either
as VGA controller (primary display) or Display controller (non-primary
display). The UEFI GOP driver depends on the device reporting a VGA
controller class code (0x030000).

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250708145211.6179-1-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

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