s390/vfio-ap: realize the VFIO_DEVICE_SET_IRQS ioctl

Realize the VFIO_DEVICE_SET_IRQS ioctl to set an eventfd file descriptor
to be used by the vfio_ap device driver to signal a device request to
userspace.

Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Tested-by: Matthew Rosato <mjrosato@linux.ibm.com>
Link: https://lore.kernel.org/r/20230530223538.279198-3-akrowiak@linux.ibm.com


Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>