vfio/igd: Update IGD passthrough documentation

vfio/igd: Update IGD passthrough documentation

A previous change made the OpRegion and LPC quirks independent of the
existing legacy mode, update the documentation accordingly. More related
topics, like creating EFI Option ROM of IGD for OVMF, how to solve the
VFIO_DMA_MAP Invalid Argument warning, as well as details on IGD memory
internals, are also added.

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250313150339.358621-1-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>