vfio/igd: Fix incorrect error propagation in vfio_pci_igd_opregion_detect()

vfio/igd: Fix incorrect error propagation in vfio_pci_igd_opregion_detect()

In vfio_pci_igd_opregion_detect(), errp will be set when the device does
not have OpRegion or is hotplugged. This errp will be propagated to
pci_qdev_realize(), which interprets it as failure, causing unexpected
termination on devices without OpRegion like SR-IOV VFs or discrete
GPUs. Fix it by not setting errp in vfio_pci_igd_opregion_detect().

This patch also checks if the device has OpRegion before hotplug status
to prevent unwanted warning messages on non-IGD devices.

Fixes: c0273e77f2d7 ("vfio/igd: Detect IGD device by OpRegion")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2968
Reported-by: Edmund Raile <edmund.raile@protonmail.com>
Link: https://lore.kernel.org/qemu-devel/30044d14-17ec-46e3-b9c3-63d27a5bde27@gmail.com
Tested-by: Edmund Raile <edmund.raile@protonmail.com>
Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Reviewed-by: Corvin Köhne <c.koehne@beckhoff.com>
Link: https://lore.kernel.org/qemu-devel/20250522151636.20001-1-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>