vfio/pci: Pass eventfd context to IRQ handler

Create a link back to the vfio_pci_core_device on the eventfd context
object to avoid lookups in the interrupt path.  The context is known
valid in the interrupt handler.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20240401195406.3720453-2-alex.williamson@redhat.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>