vfio/igd: Enable OpRegion by default

vfio/igd: Enable OpRegion by default

As the presence of OpRegion is used to detect IGD device now, and
guest driver usually depends on OpRegion to work. Enable OpRegion
on IGD devices by default for out-of-the-box passthrough experience
(except pre-boot display output), especially for libvirt users.

Example of IGD passthrough with libvirt:
<hostdev mode="subsystem" type="pci" managed="yes">
  <source>
    <address domain="0x0000" bus="0x00" slot="0x02" function="0x0"/>
  </source>
  <rom file="/path/to/igd/rom"/>
  <address type="pci" domain="0x0000" bus="0x00" slot="0x02" function="0x0"/>
 </hostdev>

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Corvin Köhne <c.koehne@beckhoff.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Tested-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250505170305.23622-7-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>