vfio/pci: introduce config_offset field in VFIOConfigMirrorQuirk

vfio/pci: introduce config_offset field in VFIOConfigMirrorQuirk

Device may only expose a specific portion of PCI config space through a
region in a BAR, such behavior is seen in igd GGC and BDSM mirrors in
BAR0. To handle these, config_offset is introduced to allow mirroring
arbitrary region in PCI config space.

Signed-off-by: Tomita Moeko <tomitamoeko@gmail.com>
Reviewed-by: Alex Williamson <alex.williamson@redhat.com>
Link: https://lore.kernel.org/r/20250104154219.7209-3-tomitamoeko@gmail.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>